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Page 1: Origin, ascent and oblique emplacement of magmas in a ... · Origin, ascent and oblique emplacement of magmas in a thickened crust: An example from the Cretaceous Fangshan adakitic

Lithos 123 (2010) 102–120

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Origin, ascent and oblique emplacement of magmas in a thickened crust: An examplefrom the Cretaceous Fangshan adakitic pluton, Beijing

Dan-Ping Yan a,⁎, Mei-Fu Zhou b, Donggao Zhao c, Jian-Wei Li d, Gen-Hou Wang a,Chang-Liang Wang a, Liang Qi b

a The State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Beijing 100083, Chinab Department of Earth Sciences, The University of Hong Kong, Pokfulam Rd., Hong Kongc Department of Geological Sciences, The University of Texas at Austin, 1 University Station C1100 Austin, TX 78712, USAd The State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan 430074, China

⁎ Corresponding author. School of Earth Sciences andGeosciences, Beijing 100083, China. Tel.: +86 10 82322

E-mail address: [email protected] (D.-P. Yan).

0024-4937/$ – see front matter © 2010 Elsevier B.V. Adoi:10.1016/j.lithos.2010.11.015

a b s t r a c t

a r t i c l e i n f o

Article history:Received 31 May 2010Accepted 26 November 2010Available online 3 December 2010

Keywords:Ellipsoidal enclavesFabricsFangshan adakitic plutonThickened crustEast China

The Fangshan pluton in the Western Hills of Beijing records a history of emplacement from primarily vertical(ascent) to oblique injection along a detachment fault. Thus, the pluton provides a classic example answeringthe “room” problem. This pluton and the Nanjiao apophyse were produced by Cretaceous magmatism thatformed the regionally extrusive East China Mesozoic igneous province. The Fangshan pluton includes twointrusive phases, a 136-Mamonzodiorite phase intruded by a 131-Ma quartz monzodiorite phase. Rocks fromboth phases have chondrite-normalized REE patterns enriched in LREE and depleted in HREE. They haveprimitive mantle-normalized trace element patterns with distinct negative Nb and Ti anomalies and high Sr(1067–1348 ppm) and low Y (8–17 ppm) with high Sr/Y ratios (71–137). These features are characteristic ofadakites and suggest that the magmas were derived by partial melting of the lower part of a thickenedcontinental crust.The pluton has magmatic foliations and lineation, indicative of the intrusion of the earlier magmas slightlyobliquely from SE to NW. The magmatic fabrics have enclave ratios Rfmag+Rp of 3.0–3.5. Asymmetric solid-state deformational fabrics and strains (Rs) for the northwestern (3.5–4.0) and southeastern parts (1.5–2.0)again suggest a later oblique injection of magmas from SE to NW.Both the Fangshan pluton and Nanjiao apophyse suggest that magmas migrated most probably along the pre-existing detachment fault. Such an emplacement is also supported by the aeromagnetic polarization data andstrong deformation of the Beiling syncline. Thus, magma emplacement in the Fangshan pluton probablyinvolved creation of space by both faulting and wallrock strain.

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1. Introduction

The so-called “roomproblem” ofmagmatic emplacement has been amatter of debate since the intrusive nature of plutons was established(Johnson et al., 1999, 2003, 2004; Vernon et al., 2004). To solve thisproblem requires better understanding of the magma origin, transportand emplacement (e.g. Antolin-Tomas et al., 2009; Petford et al., 2000;Weinberg and Regenauer-Lieb, 2010). End-member mechanisms formagma ascent and emplacement have been proposed but are hotlydebated. The classic model suggests that the magma created its ownspace through diapirism or ballooning (Buddington, 1959; He et al.,2009; Holder, 1979; Ramsay, 1989;Weinberg and Podladchikov, 1994).Castro (1987) considered tectonic factors as primary control on the

shape, emplacementmodel and geometric concordance of a pluton. Thistectonic model is supported by the emplacement of magmas along pre-existing faults (Daly, 1903; Hutton, 1982, 1988; Hutton et al., 1990;Petford and Atherton, 1992; Petronis et al., 2009; Pitcher, 1979;Weinberg and Mark, 2008; Weinberg and Regenauer-Lieb, 2010;Weinberg et al., 2004, 2009). A third model proposed by Cruden(1998) involves an end-member cantilever and piston sinking mech-anism. This model explains tubular elliptical plutons with horizontaldimensions much larger than vertical dimensions (McCaffrey andPetford, 1997; Vigneresse, 1995), and considers thatmagma ascentwaspartitioned into a low-viscosity center flowing vertically and high-viscosity outer margin flowing helically (Trubac et al., 2009). Thus,magma emplacement is probably a complex process with numerousvariations between the three main endmembers (Antolin-Tomas et al.,2009; Barbey et al., 2008; Paterson and Vernon, 1995; Petronis et al.,2009; Polteau et al., 2008; Tobisch and Williams, 1998).

The depth at which magma forms is thought to be important fordetermining the transport and emplacementmechanism (Petford et al.,

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2000), but is not well understood. In particular, how adakitic magmasformed at depth and emplaced in an extensional environment is notexplained. A representative example is the formation of adakiticmagmas in the Yanshan belt of China, which were probably producedat the base of thickened crust (~50 km) by delamination of the lowercrust at ~134 Ma (Gao et al., 2004; Rapp et al., 1999). The subsequentextensional thinning of the continental crust in this belt occurredbetween 130 and 120 Ma leading to the termination of high pressure(N1500 MPa) formation of adakitic melts (Davis, 2003).

The Mesozoic Fangshan monzodioritic pluton in the Western Hillsof Beijing is well known for its diversified and well-preservedmagmatic fabrics (Li, 1987; Shan et al., 1991), providing an excellentopportunity to study magma emplacement mechanisms. Recentinvestigations have revealed a complex history of magma emplace-ment (Shan et al., 2004; Yan et al., 2006). However, it is still unclearwhether this pluton was emplaced along a pre-existing, crustal-scaleshear zone (extension and overthrust detachment fault system; Yanet al., 2006) or by ballooning and/or diapirism (2004; He et al., 2005,2009, Shan et al., 1991). The Fangshan pluton is an ideal body forexamining the “room problem” related to the origin of graniticmagmas in the lower continental crust and their subsequentemplacement in the upper crust. In this paper, we utilize strainanalysis of enclaves and primary magmatic and solid-state fabrics,which were produced from sub-liquidus to sub-solidus state, to re-examine the emplacement mechanism of the Fangshan pluton. On thebasis of new geochemical data, we document that the pluton hasadakitic affinities and was derived from melting of the lower part of athickened crust. We further discuss the origin, ascent and emplace-ment after the magma was generated and show that the space for theintruding magma was provided by a combination of diapirism andfaulting.

2. Geological background

2.1. North China Block

The North China Block is bounded by the Qinling–Dabie orogenic beltto the south, and by the Mongol–Okhotsk accretionary belt to the north(Fig. 1A). The basement of the North China Block consists dominantly ofLate Archean to Paleo-Proterozoic gneisses, granulites, and migmatites,overlain by a variety of Mesoproterozoic to Permian cover rocks (e.g.BBGMR, 1991; Chen et al., 2009; HBG, 1989; Kusky et al., 2004; Zhao et al.,2005). The Upper Permian strata are locally overlain unconformably byLower Triassic red beds and conglomerates in the western part of theNorth China Block and the Yanshan area, whereas they are unconform-ably overlain by Lower Jurassic–Cretaceous terrestrial volcanic and clasticdeposits in the southeastern part (e.g. BBGMR, 1991; Ritts et al., 2004;Yan et al., 2006).

The collision between the South China and North China Blocksoccurred at ~230 Ma and produced the Qinling–Dabie orogenic belt(e.g., Ayers et al., 2002; Chavagnac et al., 2001; Hacker et al., 1998,2000; Yang et al., 2005). The collision was associated with formationof the sinistral, NNE-striking Tan–Lu fault (Fig. 1A; Xu et al., 1987; Renet al., 1990; Yin and Nie, 1996). In the eastern part of the North ChinaBlock, collisional indentation resulted in extensive north–southcrustal shortening and subsequent intra-continental deformation(Ren et al., 1990; Yin and Nie 1996).

The northern part of the North China Block is transected by theENE-trending Yanshan–Yinshan fold-thrust belt (BBGMR, 1991; Daviset al., 1998; HBG, 1989) (Fig. 1A). In the central part of the North ChinaBlock, the NNE-trending Taihang Mountains were uplifted in theCenozoic (BBGMR, 1991; HBG, 1989; Wang and Li, 2008). Both theYanshan–Yinshan and Taihang Mountains are marked by extensiveLate Jurassic to Early Cretaceous magmatism that produced volumi-nous intermediate-silicic intrusive and extrusive rocks (Deng et al.,2004a, 2004b).

Mesozoic granitic plutons are abundant in the Yanshan intraplateorogenic belt (YIOB) along the northeastern margin of the NorthChina Block (Fig. 1A). This igneous event has long been interpreted toresult from intraplate deformation (BBGMR, 1991; BBGMR and CUG,1988; Deng et al., 2004a, 2004b; HBG, 1989; Yan et al., 2006).Although there are numerous studies (Cai et al., 2005; Davis, 2003;Deng et al., 2004a, 2004b; Fan et al., 2003; Shan et al., 2004), thepetrogenesis of the granitic plutons and their tectonic setting are stillmatters of debate. Shan et al. (1991, 2004) and He et al. (2005, 2009)examined high-temperature shear aureole with pluton-side-upkinematic indicators, rim syncline around the pluton and magmaticfabrics, and proposed that ascent and emplacement of magmas withinthe YIOB is the result of diapirism or ballooning. However, the specialdistribution of most plutons along regional faults argues against suchan endmember emplacement mechanism (BBGMR, 1991; HBG, 1989;LBGMR, 1989; Yan et al., 2006).

2.2. Geology of the Western Hills

The Fangshan pluton is located in the Western Hills of Beijing atthe junction of the Yanshan and Taihang Ranges (Fig. 1A). Crystallinebasement rocks in the Western Hills are represented by theamphibolite facies metamorphic rocks of the Guandi complex,which are exposed both to the north and south of the Fangshanpluton. The igneous protolith of the Guandi complex has a LateArchean age of approximately 2.5 Ga (Yan et al., 2005, 2006) (Fig. 1B).The basement rocks are tectonically overlain by Mesoproterozoicstrata of the Changcheng and Jixian systems (Fig. 1B). The Mesopro-terozoic sequence is conformably overlain by Neoproterozoic toLower Paleozoic strata, and Upper Paleozoic, Triassic and Jurassicsequences form the core of the Beiling syncline, west of the Fangshanpluton. In the Nanjiao area, west of the Beiling syncline, several quartzmonzodiorite apophyses, which have zircon LA-ICP-MS U–Pb ages of133.0 Ma and 134.0 Ma (Zhang et al., 2008a), are petrologicallysimilar to the Fangshan pluton and intrude along pre-existing and re-activated, east-dipping, low-angle faults within the lower Paleozoicsection (Shan et al., 1991; Zhang et al., 2008a) (Fig. 1). In thenortheastern segment of the Fangshan pluton, small quartz mon-zodiorite apophyses intrude the Nandazai thrust fault (Fig. 1B).

The Guandi complex forms the core of a typical extensionaltectonic dome, intruded by the Early Cretaceous Fangshan pluton(Figs. 1 and 2). Structures and fabrics from the Guandi core complexand the Fangshan pluton define at least 5 stages (D1–D5) ofdeformation and magmatic–tectonic events (Shan et al., 1991; Yanet al., 2006).

The earliest deformation (D1) is characterized by a series of Middleto Late Triassic, low-angle detachment faults, which separate theGuandi core complex, the Neoproterozoic Jixian and Qinbaikousystems, and the Cambrian–Ordovician and Carboniferous–Permianrocks from the overlying strata (BBGMR, 1991; Yan et al., 2006). D2 isdefined by numerous Late Triassic to Jurassic east–west-strikingthrust faults and folds (Fig. 1B–C) (Yan et al., 2006). Immediatelyfollowing D2, WNW-directed thrusting along the Nandazai fault (D3)overprinted D1 and D2. Thrust faults of both D2 and D3 cut D1

structures in the upper crust, and together with the deeper masterdetachment above the Guandi complex, form the detachment faultsystem (Shan et al., 1991; Yan et al., 2006).

Structures associated with the emplacement of plutons including theFangshan pluton in the area define D4, resulting in the formation of thecurved Beiling syncline, which is superimposed on a pre-existing, east–west-striking syncline originally formed by D2. Granodioritic apophysesintruded along theNandazhai thrust fault (D3) suggest that D4 (Fig. 1B–C;Yan et al., 2006) is coeval with the gravitational collapse of orogenicallythickened crust in concordance with the development of isolatedmetamorphic core complexes identified by Davis et al. (1996, 1998,2002) and Darby et al. (2001). A late-stage (Late Cretaceous to

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Quaternary), NE-trending, high-angle normal fault system (D5) separatesthe North China Plain from the Western Hills, and crosscuts all earlierstructures (Fig. 1B–C) (Menget al., 2003; Renet al., 2002; Yanet al., 2006).

2.3. Field relations and petrography of the Fangshan pluton

The Fangshan pluton is a NW-striking ellipsoid with an outcrop areaof 52 km2, composed of two nested intrusive phases of monzodioriteand quartz monzodiorite (Fig. 2) (BBGMR, 1991; Li, 1987).

The monzodiorite phase forms the northeastern, southeastern andsouthwestern parts of the pluton and is intruded by the quartzmonzodiorite phase (Fig. 2). A fine-grained chilled margin can beobserved in the quartz monzodiorite, which is sub-parallel to themagmatic fabric (Fig. 3A). The monzodiorite is a medium-grained(b1.0 mm),weakly plagioclase-phyric (5%) rock composed of about 55%plagioclase (An33–42), 9–12% amphibole, 10% quartz, 8% K-feldspar, 6–13% biotite, and 1–2% apatite and zircon combined. The medium- tocoarse-grained (N1.0 mm) quartz monzodiorite is only locally andsparsely porphyritic,with up to3%plagioclase phenocrysts. It consists of37–42% plagioclase (An20–26), 20% each of quartz and K-feldspar, 9%amphibole, 7% biotite, and 1–2% accessory minerals (Fig. 3A). Both themonzodiorite and quartz monzodiorite phases have abundant dark,dioritic enclaves (Fig. 3B, C) and wall-rock xenoliths. The quartzmonzodiorite phase varies slightly from its margins to its core (Fig. 2),gradually increasing the grain size frommedium- to coarse-grained, anddecreasing its abundance of mafic minerals, wall-rock xenoliths andmafic enclaves. Some amphibole-bearing lamprophyric dikes andelliptical intrusions of intermediate to mafic composition are locatedalong the contact between the two intrusions (Fig. 2).

3. Geochronology and geochemistry

3.1. Analytical methods

3.1.1. Ar–Ar datingTwo samples (Fg-1 and Fg-2, collected at 39°42′19″N, 115°57′41″E

and 39°45′31″N, 115°58′27″E, respectively) from the monzodioritephase were selected for hornblende separates. Both samples are freshand contain magmatic hornblende suitable for 40Ar/39Ar dating. Thesamples were crushed, washed in distilled water in an ultrasonic bathfor 1 h, and then dried. Hornblende separates (1–2 mm long) werehandpicked under a binocular microscope. Mineral separates wereirradiated along with monitors of the Fish Canyon sanidine (28.02 Ma;Lamphere and Baadsgaard, 1997) and ZHB biotite (133.3 Ma; Fu et al.,1987) for 9 h at the radiation center, Institute of Atomic Energy of China.

After two months of cooling, 32–34 mg of hornblende separatesfrom each sample were analyzed by incremental heating employing aMM-5400 mass spectrometer at the China University of Geosciences,Beijing. Argon gas was extracted at consecutively higher temperaturesfor 10 min at each designated temperature. Measured argon isotopepeak heights were extrapolated to zero time, normalized to the 40Ar/36Ar atmospheric ratio (295.5) using measured values of atmosphericargon, and corrected for neutron-induced 40Ar from K, 39Ar and 36Arfrom calcium, and 36Ar from chlorine (Onstott and Peacock, 1987). Thevalues for the reactor correction factors are: (40Ar/39Ar)K=(5.84±0.89)×10− 3, (36Ar/37Ar)Ca= (2.78±0.079)×10− 4, and (39Ar/37Ar)Ca=(7.26±0.08)×10−4. The J factor used in the age calculationsis 0.00142.

3.1.2. Geochemical analysesThirty-one fresh whole-rock samples were collected from the

Fangshan pluton for major oxide and trace element analyses. Samples

Fig. 1. Regional geological map of the Fangshan area, southwestern Beijing. Map A shows theet al. (2006) and BBGMR and CUB (1988). D1–D5 represents deformation stages identified byRange; QDOB: Qinling–Dabie Orogenic Belt; SGT: Songpan–Ganze Terrane; SLOB: Sulu Orog

were cut with a diamond-impregnated brass blade, crushed in a steeljaw crusher that was brushed and cleaned with de-ionized waterbetween samples, and pulverized in an agate mortar. Major oxideswere determined by wavelength-dispersive X-ray fluorescencespectrometry (WD-XRFS) on fused glass beads using a PhilipsPW2400 spectrometer at The University of Hong Kong. Traceelements were determined by inductively coupled plasma-massspectrometry (ICP-MS) of nebulized solutions using a VG Plasma-Quad Excell ICP-MS at The University of Hong Kong after a 2-dayclosed beaker digestion using a mixture of HF and HNO3 acids in high-pressure vessels. Pure elemental standard solutions were used forexternal calibration and BHVO-1 and SY-4 were used as referencematerials. The accuracies of the XRF analyses are estimated to be ±1%(relative) for SiO2, ±2% (relative) for other major oxides present inconcentrations greater than 0.5 wt.% and ±5% (relative) for minoroxides present in concentrations greater than 0.1%. The accuracies ofthe ICP-MS analyses are estimated to be better than±5% (relative) formost elements (Zhou et al., 2006).

3.2. Analytical results

3.2.1. 40Ar/39Ar agesThe analytical results are listed in Table 1, and illustrated in Fig. 4. All

data reported are quoted at 1σ error, which include the analyticaluncertainties of themonitor analyses (J-values), but the error for the ageof the monitor is assumed to be zero. In this study, an age plateau isdefined using the criteria of McDougal and Harrison (1999), wherethree or more consecutive steps, corresponding to at least 50% of thetotal 39Ar released, yield apparent ages reproducible at the 95%confidence level.

The analyzed samples yieldedwell-defined plateau ages of 134.3±1.4 Ma and 136.0±1.5 Ma (Fig. 4). Sample Fg-1 has an isochron age of135.6±2.9 Ma, which is consistent with the corresponding plateauage within the analytical uncertainty (Fig. 4). However, sample Fg-2yielded an isochron age slightly different from its plateau age(Fig. 4). The isochrons have initial 40Ar/36Ar ratios of 293.0±8.0 and313.0±11.0, indistinguishable from that of the present-day atmo-sphere, indicating that there was no incorporation of excess argon inthe samples. We interpret the plateau ages to represent the coolingage of the monzodiorite below ca. 550 °C (McDougal and Harrison,1999).

3.2.2. Major element oxides and trace elementsSamples from both the quartz monzodiorite and monzodiorite

phases have similar chemical composition with SiO2 contents rangingfrom 57.9 to 65.3 wt.%, Al2O3 from 15.5 to 18.4 wt.%, and MgO from 1.5to 3.1 wt.% (Table 2). The rocks are K-rich, with Na2O ranging from4.04 to 4.64 wt.% and K2O from 3.28 to 4.93 wt.%, yielding K2O/Na2Oratios of 0.74–1.1. They have high Sr (1067 to 1348 ppm) but low Yconcentrations (8–17 ppm) with high Sr/Y ratios (71–137) (Table 2).In the Y vs. Sr/Y and YbN vs. (La/Yb)N diagrams, all samples plot in theadakitic field (Fig. 5A and B). Chondrite-normalized REE patterns ofthese samples are generally depleted in heavy REE (HREE) andenriched in light REE (LREE) (Fig. 6A). They also have low YbN (mostlyb4) and high (La/Yb)N values that plot in the field of adakite (Fig. 5B).Both the monzodiorite and quartz monzodiorite are similar to high-Siadakites (HSA) (Fig. 5C–F), as defined by Martin et al. (2005). In theprimitive mantle-normalized trace element spider diagram, the rocksshow obvious negative Nb and Ti anomalies, but positive Sr and Baanomalies (Fig. 6B). Their low Rb/Sr ratios are similar to those ofadakitic plutons in eastern China and Cenozoic adakites worldwide(Table 2).

tectonic location of the Fangshan area within the North China Block (modified from YanYan et al. (2006). T.H. Mt.: Taihang Mountain Range; Yanshan Mt.: Yanshan Mountainenic Belt; TLF: Tan–Lu Fault.

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Fig. 2. Geological map of the Fangshan pluton (area outlined in Fig. 1B), showing the twomagmatic units of quartz monzodiorite (Qmd) andmonzodiorite (Md), themagmatic foliation and lineation and the distribution of the other rock units.Magmatic foliation projections (equal area, lower hemisphere) are pole diagrams for the northwestern, northeastern, southwestern and southeastern parts of the pluton. Note that the pluton exhibits solid-state deformational fabrics only inits northwestern area; weak fabrics are locally present.

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4. Fabrics of the Fangshan pluton

4.1. Primary magmatic structures and solid-state fabrics of the enclavesand host rocks

The orientation of long crystal planar of biotite and amphibolecrystals, K-feldspar laths, plagioclase megacrysts, and flattenedsurface of elongate enclaves defines magmatic foliation (Smag),whereas their maximum-crystal or -strain axis defines the lineation

Fig. 3. Field photos of the rock units and ellipsoidal dioritic enclaves. (A) Quartz monzodior(B) ellipsoidal dioritic enclave. Note that the foliation within the enclave is consistent with thnot clear, the angle of maximum fluctuation Fθmag is defined by the long axis of the enclav(D) outcrop along the joint sets 1, 2 and 3 indicating by the magmatic lineation and foliatioutcrop section; (F) The mafic and/or intermediate enclave within the felsic host rocks pro

(Lmag) on the Smag (Fig. 3B and E). Both Smag and Lmag are subtlydeveloped but common in the Fangshan pluton (Figs. 3A–C and 7A). Ingeneral, the primary structures are more evident in the monzodioritethan in the quartz monzodiorite. The Smag is better defined than theLmag, and generally parallels the boundary between the two units andthe margins of the pluton. The magmatic foliation dips outward in thenortheastern, southeastern and southwestern parts of the pluton atangles of 65–85°, but inward with steep angles of 80–90° in thenorthwestern part of the pluton (Fig. 2). The magmatic lineation has a

ite intruded into monzodiorite with a chilled margin on the quartz monzodiorite side;at in the host rocks; (C) ellipsoidal dioritic enclaves. Although the magmatic foliation ises, and thus the bisectrix of the Fθmag defines the Smag (Tobisch and Williams, 1998);on; (E) a model showing that X, Y and Z are directly measured or estimated along thebably produced by magma mingling.

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Fig. 4. 40Ar/39Ar ages of the amphibole separates from the early monzodiorite unit of the Fangshan pluton. For sample locations see Fig. 2.

Table 140Ar/39Ar results of hornblende from the monzodiorite, Fangshan pluton.

T (°C) 40Ar/39Ar 36Ar/39Ar(×10−3)

37Ar/39Ar 40Ar*/39Ar 39Ar (×10−12)(mol)

39Ar released(%)

38Ar/39Ar 40Ar/40Ar(%)

Age (Ma) ±(Ma)

Sample Fg-1, J=0.00142±0.0000232, Wt=0.0334 (g)650 1081.2490 4.101109 1609.36651 55.8170 0.004 0.53 1.0843 0.17 137.60 592.94720 989.2541 3.034576 471.06988 202.9836 0.011 1.37 4.2678 13.88 456.91 539.57790 1146.4752 3.234177 33.24432 198.4443 0.002 0.20 7.9911 17.93 447.86 749.99850 688.0585 1.797940 541.60459 346.3259 0.001 0.18 5.2541 29.32 721.55 809.08950 880.8887 2.781783 89.30110 70.2518 0.006 0.67 5.5318 8.61 171.54 175.361000 594.4320 1.837469 7.80622 52.3376 0.009 1.02 4.2548 9.94 129.32 79.801050 257.3395 0.728923 145.94733 59.2503 0.020 2.36 1.5013 21.37 145.73 19.261100 482.1724 1.527989 147.71392 46.6712 0.017 2.01 3.5071 9.73 115.76 38.831200 76.5382 0.076741 9.66844 54.9700 0.342 41.06 1.5592 71.64 135.59 2.041300 93.9884 0.140174 11.70150 53.8998 0.397 47.64 1.8542 57.37 133.05 2.061420 585.4132 1.850966 67.56802 45.7265 0.025 2.95 4.9374 8.60 113.49 27.85

Sample Fg-2, J=0.00142±0.0000232, Wt=0.032 (g)650 335.2519 0.879875 879.45540 472.4789 0.002 0.26 1.2875 41.86 925.87 696.28780 234.8658 0.912155 1038.48511 238.7477 0.004 0.45 5.7513 17.76 526.50 126.90880 395.8862 0.713243 475.44310 354.7548 0.003 0.32 3.0406 55.88 735.75 202.22940 562.1248 1.791311 572.71010 136.2801 0.006 0.65 1.3148 14.21 319.03 103.771000 313.2027 0.882954 127.37851 68.3266 0.011 1.26 2.5267 20.64 167.00 34.601050 298.6689 0.886971 356.00183 86.6573 0.016 1.90 2.4847 21.75 209.29 35.321100 396.0815 1.255756 459.96627 91.5290 0.021 2.40 3.6651 15.68 220.36 51.791150 321.3964 0.927673 146.46729 65.3595 0.031 3.60 3.2034 19.02 160.06 20.741200 91.4542 0.125927 9.29673 55.3112 0.287 33.53 2.1923 60.55 136.35 2.331300 94.4267 0.138160 9.81704 54.7244 0.461 53.83 2.3171 58.05 134.96 2.091420 868.5375 2.872250 307.94854 55.3419 0.015 1.80 7.6598 6.04 136.43 47.49

40Ar* denotes radiogenic 40Ar.

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Table 2Major and trace elements of the Fangshan pluton, Western Hills of Beijing.

Monzodiorite

FS-1 FS-2 FS-4 FS-4A FS-6 FS-7 FS-8 FS-11 FS-12 FS-13 FS-14 FS-21 FS-22 FS-23 FS-24

Major oxides (wt.%)SiO2 65.31 62.27 62.15 62.99 62.72 63.62 59.23 62.65 64.24 63.23 63.22 61.49 61.81 62.15 62.37TiO2 0.60 0.67 0.63 0.64 0.66 0.63 0.58 0.67 0.59 0.59 0.60 0.66 0.66 0.64 0.66Al2O3 15.54 15.87 16.05 16.24 16.50 16.11 18.36 16.33 16.11 16.11 16.49 16.74 16.37 16.53 16.30Fe2O3 4.08 5.27 4.71 4.77 4.86 4.63 4.43 4.96 4.30 4.44 4.36 5.07 5.08 5.13 5.25MnO 0.05 0.08 0.07 0.07 0.07 0.07 0.08 0.07 0.06 0.06 0.06 0.08 0.08 0.08 0.08MgO 1.50 2.11 1.96 1.98 2.06 1.95 1.95 2.09 1.82 1.83 1.77 2.03 2.08 2.08 2.14Mg# 39.59 41.64 42.59 42.52 43.04 42.88 43.96 42.89 43.00 42.35 41.98 41.65 42.19 41.95 42.08CaO 3.01 3.92 3.72 3.77 3.96 3.76 3.95 3.89 3.52 3.59 3.67 4.18 4.12 4.17 4.07Na2O 4.41 4.20 4.22 4.24 4.37 4.44 4.64 4.37 4.04 4.36 4.45 4.47 4.28 4.30 4.19K2O 3.53 3.36 3.75 3.57 3.52 3.28 4.93 3.28 3.98 3.74 3.65 3.37 3.56 3.46 3.51P2O5 0.25 0.31 0.29 0.29 0.31 0.28 0.30 0.31 0.31 0.29 0.28 0.32 0.32 0.32 0.33LOI 0.87 0.74 0.79 0.77 0.79 0.87 0.97 0.90 0.66 0.80 0.79 0.99 0.83 1.06 0.83TOT 99.15 98.80 98.32 99.33 99.82 99.63 99.43 99.53 99.63 99.03 99.35 99.38 99.20 99.92 99.72

Trace elements (ppm)Li 19.96 19.30 18.62 18.93 20.59 15.64 20.99 20.94 17.88 23.76 16.49 17.50 19.02 18.17Be 1.51 1.78 1.86 1.95 1.92 2.69 1.80 1.71 1.84 1.77 1.92 1.88 1.88 1.86Sc 8.91 10.44 10.17 9.90 10.21 13.08 10.64 9.85 10.37 10.13 11.58 11.42 11.00 10.58V 66.97 89.19 80.77 83.92 80.68 76.56 84.65 72.33 76.86 76.82 86.24 86.83 85.42 87.15Cr 20.23 23.09 24.54 26.91 24.92 29.90 24.62 21.26 23.27 22.75 18.34 19.21 24.00 20.54Co 9.00 12.39 11.44 11.85 11.43 10.37 12.02 10.20 10.77 10.44 11.89 11.89 11.90 12.24Ni 11.29 14.05 12.97 13.79 12.50 14.59 12.82 9.89 11.99 10.83 11.91 10.55 14.10 11.78Cu 9.54 8.31 8.21 8.21 7.40 16.21 7.84 6.64 7.54 7.24 8.05 7.72 10.22 7.95Zn 74.44 96.81 79.57 83.32 70.85 107.94 85.69 87.61 82.53 72.23 85.17 76.99 103.59 89.66Ga 22.49 22.34 22.25 22.38 22.48 24.32 22.68 21.22 21.92 22.47 22.15 21.86 22.04 21.67Ge 1.53 1.84 1.59 1.68 1.70 1.97 1.78 1.49 1.66 1.64 1.75 1.78 1.75 1.75Rb 64.48 69.97 75.43 72.89 72.21 96.38 71.25 85.36 71.95 70.75 65.83 72.60 73.65 72.90Sr 1119.25 1099.49 1143.83 1142.99 1127.57 1099.96 1134.20 1129.89 1137.77 1219.36 1175.60 1157.20 1161.83 1114.90Y 8.17 14.39 12.12 12.17 11.95 12.26 12.59 10.51 10.45 10.84 13.31 14.77 13.46 13.83Zr 185.64 200.87 182.07 179.02 172.52 177.31 187.37 169.80 170.90 182.17 192.28 184.94 191.36 178.42Nb 8.38 9.68 8.71 9.16 8.80 9.99 9.38 8.36 8.18 8.50 9.20 9.21 8.21 8.75Mo 0.37 0.53 0.46 0.59 0.54 0.85 0.43 0.68 0.48 0.50 0.62 0.56 0.83 0.56Cd 0.13 0.89 0.14 0.20 0.11 1.44 0.15 0.61 0.34 0.18 0.26 0.11 3.43 0.43Sn 2.70 27.33 3.08 5.08 3.01 44.34 3.28 17.75 9.05 4.44 6.18 2.44 109.40 11.35Sb 0.14 0.16 0.07 0.06 0.06 0.48 0.05 0.22 0.13 0.08 0.31 0.10 0.37 0.14Cs 1.59 0.90 1.05 1.14 0.96 1.46 1.04 1.15 0.83 0.93 0.91 1.01 0.92 0.94Ba 1740.99 1434.57 1688.43 1694.30 1469.57 1555.30 1532.46 2074.81 1875.69 1989.14 1666.82 1764.01 1611.48 1734.37La 40.41 54.56 47.82 49.04 58.51 58.42 52.50 48.58 62.37 65.43 54.33 59.09 66.94 55.69Ce 85.26 103.13 93.13 97.51 107.96 109.10 105.50 94.26 109.29 116.50 101.36 115.01 123.52 106.30Pr 9.88 11.50 10.46 10.57 11.16 10.98 11.55 10.38 11.07 11.72 10.88 12.25 12.51 11.16Nd 36.94 41.16 37.57 39.77 39.90 39.12 43.28 36.82 39.74 41.17 40.63 44.46 44.11 40.96Sm 5.76 6.54 6.13 6.28 5.99 6.18 6.57 5.95 5.97 6.00 6.44 7.14 6.74 6.62Eu 1.44 1.56 1.60 1.62 1.57 1.58 1.65 1.58 1.53 1.63 1.74 1.80 1.58 1.66Gd 4.97 6.18 5.47 5.88 5.86 5.82 6.11 5.64 5.50 5.71 5.94 6.57 6.74 6.19Tb 0.47 0.61 0.54 0.55 0.57 0.56 0.59 0.52 0.51 0.53 0.60 0.66 0.63 0.61Dy 1.79 2.60 2.33 2.39 2.32 2.38 2.51 2.21 2.24 2.24 2.66 2.88 2.74 2.76Ho 0.29 0.48 0.42 0.44 0.40 0.43 0.44 0.38 0.38 0.38 0.46 0.53 0.49 0.48Er 0.85 1.41 1.25 1.29 1.26 1.30 1.35 1.16 1.08 1.16 1.45 1.64 1.52 1.51Tm 0.10 0.19 0.15 0.16 0.16 0.16 0.17 0.14 0.14 0.14 0.19 0.19 0.19 0.20Yb 0.62 1.24 1.04 1.05 1.08 1.12 1.09 0.93 0.97 0.93 1.29 1.34 1.26 1.26Lu 0.09 0.18 0.16 0.15 0.15 0.17 0.15 0.14 0.14 0.14 0.20 0.20 0.19 0.18Hf 4.53 4.61 4.64 4.53 4.51 4.87 4.71 4.09 4.18 4.30 4.75 4.55 4.60 4.36Ta 0.47 0.58 0.54 0.54 0.53 0.50 0.56 0.53 0.50 0.49 0.48 0.53 0.39 0.45W 0.00 0.09 0.06 0.04 0.09 0.19 0.01 0.04 0.01 0.01 0.07 0.07 0.07 0.10Tl 0.32 0.35 0.41 0.37 0.37 0.44 0.38 0.46 0.40 0.37 0.35 0.39 0.39 0.40Pb 17.66 17.72 19.17 18.64 18.62 32.70 17.86 21.29 19.43 19.22 18.92 19.11 21.17 18.80Bi 0.09 0.15 0.07 0.07 0.06 0.15 0.05 0.04 0.04 0.05 0.06 0.06 0.07 0.09Th 7.48 9.91 8.33 10.02 10.49 29.96 9.56 8.99 9.79 10.49 11.82 12.60 14.01 9.50U 1.02 1.53 1.63 1.84 1.83 4.12 1.73 1.43 1.63 1.72 2.39 2.30 2.00 1.94

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Table 2 (continued)

Monzodiorite Quartz monzodiorite

FS-1 FS-25 FS-26 FS-27 FS-28 FS-30 FS-31 FS-32 FS-33 FS-35 FS-36 FS-37 FS-17 FS-18 FS-18A FS-19 FS-20

Major oxides (wt.%)SiO2 62.48 62.27 61.55 62.29 62.63 62.35 62.71 62.28 62.67 63.15 62.85 57.85 58.20 58.87 57.67 59.77TiO2 0.65 0.60 0.70 0.65 0.65 0.68 0.65 0.64 0.61 0.56 0.58 0.71 0.72 0.72 0.76 0.78Al2O3 15.91 16.10 16.39 15.98 15.81 16.00 16.39 16.10 15.89 16.14 16.24 17.35 17.22 17.44 16.57 16.99Fe2O3 4.83 4.75 5.04 4.82 4.81 5.11 4.96 4.79 4.82 4.47 4.71 6.15 6.14 6.20 6.68 6.13MnO 0.07 0.07 0.07 0.07 0.07 0.07 0.08 0.08 0.08 0.07 0.07 0.10 0.11 0.11 0.11 0.09MgO 2.01 1.99 2.11 2.00 2.01 2.12 2.02 2.05 1.97 1.84 1.94 2.68 2.72 2.76 3.08 2.63Mg# 42.59 42.75 42.73 42.52 42.69 42.51 42.06 43.27 42.15 42.32 42.34 43.72 44.12 44.24 45.11 43.33CaO 3.81 3.61 3.95 3.87 3.60 3.82 3.89 3.77 3.64 3.46 3.57 5.23 5.12 5.19 5.40 4.63Na2O 4.24 4.20 4.35 4.37 4.17 4.31 4.26 4.22 4.01 4.07 4.04 4.26 4.26 4.24 3.94 4.27K2O 3.68 3.75 3.27 3.41 3.70 3.49 3.44 3.45 3.72 4.01 3.93 3.32 3.35 3.26 3.40 3.49P2O5 0.30 0.30 0.32 0.29 0.30 0.31 0.29 0.29 0.29 0.27 0.29 0.36 0.36 0.37 0.39 0.37LOI 0.87 0.82 0.80 1.16 0.97 1.05 0.69 0.99 0.87 0.89 0.94 1.01 1.11 1.08 1.24 1.07TOT 98.86 98.43 98.56 98.90 98.72 99.32 99.36 98.65 98.56 98.94 99.18 99.02 99.30 100.23 99.25 100.23

Trace elements (ppm)Li 18.84 18.23 17.67 20.09 18.02 19.64 20.32 20.44 21.03 19.75 19.91 12.18 12.36 14.74 17.12Be 1.73 1.76 1.77 1.99 1.71 1.72 1.78 1.78 1.77 1.79 1.61 1.44 1.34 1.53 1.72Sc 10.51 10.34 11.15 12.68 11.43 10.65 10.60 10.82 11.25 10.23 10.77 10.23 11.78 15.43 12.10V 86.02 81.70 88.89 93.08 83.61 86.34 84.13 80.86 81.17 73.50 78.71 107.93 106.62 119.51 105.82Cr 24.13 23.57 29.72 30.84 23.95 23.90 22.88 20.90 21.40 20.79 22.08 26.95 26.07 34.81 27.68Co 11.71 11.40 12.75 13.05 12.04 12.41 11.89 11.47 11.18 10.42 11.26 14.49 14.73 16.74 15.09Ni 11.37 11.40 15.25 14.42 39.65 11.88 18.14 11.05 11.71 11.31 11.35 12.26 14.34 22.32 14.23Cu 7.93 7.28 11.17 9.48 35.69 8.97 10.68 10.18 9.77 9.25 9.98 13.82 14.73 16.12 9.39Zn 84.75 86.26 110.58 94.30 87.64 84.88 83.86 89.69 81.19 78.36 78.98 88.31 90.20 112.17 98.16Ga 22.04 21.97 22.98 24.87 22.21 23.55 22.97 22.35 21.49 20.91 21.46 22.10 21.65 22.50 22.51Ge 1.70 1.67 1.69 1.92 1.76 1.78 1.78 1.81 1.66 1.67 1.70 1.80 1.82 1.95 1.79Rb 75.15 74.14 71.85 79.91 75.03 76.32 72.75 74.17 82.46 81.58 85.94 58.22 58.52 59.97 72.99Sr 1113.82 1126.33 1195.26 1218.33 1092.50 1186.17 1146.17 1110.48 1067.97 1092.41 1117.35 1347.56 1315.79 1220.16 1245.20Y 12.80 10.92 11.96 12.52 13.41 13.18 13.40 14.02 12.66 11.85 11.83 14.80 14.24 17.31 14.88Zr 177.88 170.91 189.26 179.10 160.77 162.31 148.24 156.35 147.49 141.46 143.83 161.63 172.96 201.97 212.22Nb 9.66 7.71 9.34 9.92 9.77 9.84 9.82 10.44 8.93 8.76 7.93 7.69 7.57 8.65 9.05Mo 0.54 0.42 0.89 0.52 0.54 0.51 0.65 0.66 0.49 0.52 0.47 0.92 0.93 1.10 0.61Cd 0.29 0.23 1.83 0.14 0.24 0.15 0.21 0.37 0.18 0.29 0.24 0.29 0.17 0.48 0.18Sn 7.51 5.44 57.29 2.58 6.69 3.12 5.62 8.82 3.17 6.43 5.59 6.94 2.76 11.02 4.45Sb 0.06 0.07 0.29 0.14 0.08 0.07 0.10 0.07 0.06 0.07 0.08 0.06 0.94 0.34 0.09Cs 1.20 1.23 0.78 1.30 1.14 1.01 1.00 1.11 1.10 0.92 0.96 1.22 1.00 1.05 1.23Ba 1722.72 1836.59 1753.85 1634.88 1603.91 1490.94 1569.82 1570.95 1547.83 1823.50 1911.11 2125.07 2040.53 2032.81 1895.89La 51.82 49.29 50.00 68.50 46.67 57.86 55.02 58.17 49.31 48.19 57.93 54.66 48.99 54.58 40.67Ce 106.13 95.71 100.36 125.96 96.83 112.83 106.32 109.63 97.76 96.56 102.89 99.59 94.48 105.76 85.90Pr 11.62 10.23 11.33 13.07 10.96 12.25 11.23 11.97 10.39 10.03 10.55 10.92 10.53 12.01 10.02Nd 43.70 38.57 40.87 46.46 43.40 43.28 40.37 42.66 37.79 35.37 37.16 40.68 38.97 44.86 39.63Sm 6.77 5.94 6.54 6.82 7.17 6.53 6.26 6.72 6.16 5.76 5.60 6.49 6.32 7.57 6.80Eu 1.67 1.52 1.75 1.73 1.64 1.68 1.62 1.66 1.55 1.49 1.43 1.88 1.80 1.98 1.75Gd 6.28 5.68 6.16 6.59 6.10 6.26 5.91 6.26 5.51 5.48 5.44 6.26 5.98 6.86 6.13Tb 0.61 0.53 0.59 0.62 0.62 0.58 0.57 0.60 0.57 0.55 0.52 0.66 0.60 0.73 0.62Dy 2.57 2.20 2.56 2.59 2.72 2.48 2.45 2.59 2.50 2.27 2.21 2.89 2.77 3.32 2.81Ho 0.46 0.38 0.43 0.44 0.46 0.43 0.44 0.48 0.45 0.42 0.41 0.55 0.50 0.64 0.53Er 1.36 1.15 1.28 1.35 1.36 1.32 1.39 1.46 1.31 1.26 1.19 1.67 1.56 1.96 1.59Tm 0.17 0.15 0.16 0.16 0.17 0.15 0.17 0.18 0.17 0.16 0.16 0.20 0.21 0.26 0.21Yb 1.16 0.93 1.04 1.11 1.15 1.02 1.14 1.21 1.17 1.12 1.05 1.42 1.33 1.70 1.38Lu 0.17 0.14 0.15 0.17 0.17 0.15 0.17 0.18 0.17 0.15 0.15 0.22 0.20 0.26 0.21Hf 4.31 4.25 4.57 4.51 3.92 3.90 3.56 3.83 3.73 3.58 3.49 3.42 3.95 4.59 5.10Ta 0.62 0.46 0.55 0.56 0.62 0.56 0.60 0.63 0.56 0.52 0.43 0.40 0.40 0.47 0.51W 0.07 0.12 0.08 0.07 0.04 0.03 0.05 0.05 0.02 0.02 0.04 0.14 0.16 0.34 0.11Tl 0.39 0.39 0.41 0.44 0.40 0.40 0.37 0.38 0.43 0.42 0.45 0.33 0.33 0.31 0.42Pb 19.56 18.51 20.41 21.98 19.20 19.13 17.77 18.75 19.88 21.01 21.86 18.25 19.90 19.53 17.82Bi 0.07 0.07 0.06 0.07 0.08 0.05 0.07 0.07 0.05 0.05 0.05 0.13 0.09 0.11 0.07Th 9.34 9.73 11.48 12.48 9.97 9.29 12.36 12.23 11.52 9.99 10.17 8.14 6.62 8.51 6.38U 1.76 1.76 1.80 2.16 1.84 1.55 2.04 1.77 1.77 1.50 1.34 2.25 1.84 2.25 1.68

Mg#={[(MgO/40.304)/(MgO/40.304)+(Fe2O3/71.839)]⁎100}.

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Fig. 5. Plots of trace elements. A: Y versus Sr/Y, B: (Yb)N versus (La/Yb)N, C: MgOwt.% versus SiO2 wt.%, D: K versus Rb, E: Cr/Ni versus TiO2 wt.%, F: Sr/Y versus Y. For sample locationssee Fig. 2.

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more consistent orientation, plunging east to southeast at angles of50–85° (Fig. 2).

Solid-state fabrics, including foliation, lineation and ductileshear zones, which are defined by deformational fabrics showingre-crystallization, are well developed in the northwestern part ofthe pluton, poorly developed in the southeastern part, and rare inits central, northeastern and southwestern parts (Figs. 7 and 8A–D). In the northwestern part of the pluton, the quartz monzodioriteis gneissic and the shapes of the enclaves change from angularto discoidal, producing a well-developed foliation (Song, 1996;Song et al., 1996; Yan et al., 2006; Zhang and Li, 1991) (Figs. 2, 7and 8).

The igneous rocks are locally mylonitized in the northwest andsoutheast parts of the pluton along two sets of conjugate, ductile shearzones (Figs. 7 and 8A–D). One set is vertical, strikes ENE and has adextral shear sense, whereas the other set strikes NNW and has asinistral sense of shear (Figs. 7 and 8A–D). Both sets of shear zoneshave S–C fabrics and share a common orientation for the S plane.

Both S and C foliations are steeply dipping. The S foliation strikes NEwhereas the C foliation strikes NNW and ENE (Figs. 7A–E and 8B–C).There is no discernable difference between S and C foliations for proto-mylonite with large plagioclase porphyroclasts (Fig. 8D). On the Sfoliation there is an ENE-plunge mineral lineation with a gentle plunge(Figs. 7F and 8D). The bisectrix of the obtuse angle of C foliations in the

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Fig. 6. (A) Chondrite-normalized REE patterns of the Fangshan plutonic rocks; (B) N-MORB-normalized trace element patterns showing the identification of high-SiO2 adakites (HSA) andlow-SiO2 adakites (LSA) (Martin et al., 2005) for dioritic rocks from the Fangshan pluton. Normalization values are from Sun and McDonough (1989). For sample locations see Fig. 2.

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conjugate shear zones strikes northwest–southeast, indicating that themaximum contraction direction σ1 is northwest, which coincides inorientation with the normal of the D4 syn-deformational S foliation(Fig. 8A–C). As a result of this contraction, the Beiling syncline, whichhad an original east–west-striking axial plane formed during D2 north–south compression, wasmost probably re-deformed by the intrusion toform an arcuatewedge to the northwest (Fig. 1B) (Shan et al., 1991; Yanet al., 2006).

There are three sets of well-developed primary joints. Set 1 isperpendicular to Smag and parallel to Lmag, whereas set 2 is parallel toboth Smag and Lmag, and set 3 is perpendicular to both Smag and Lmag

(Fig. 3D–E), thus joint sets 1, 2 and 3 are perpendicular to each other(Fig. 3E). Pegmatitic, aplitic and dioritic dikes are present along someprimary joints with radiating strikes. These dikes crosscut themagmatic foliation and lineation (Fig. 2). Well-developed, NNW-striking felsic and dioritic dikes in the marginal part of the quartzmonzodiorite range in width from a few cm to 80 cm (Fig. 3D). Someof these dikes crosscut sedimentary strata in the wallrock. Locally,subordinate NE-striking dikes are also present (Fig. 2). Nearly N–Sstriking brittle deformational zones, which contain a variety of jointsand small faults, cut both the monzodiorite and quartz monzodioriteand some of the dikes, indicating that they are coincident with thehigh-angle normal faulting of D5 (Fig. 2) (Yan et al., 2006).

4.2. Semi-quantitative estimate of primarymagmatic and solid-state strain

A variety of xenoliths with roughly ellipsoidal shapes occur alongthe margins of the pluton. These show variable rheological contrastwith the host rocks, making it difficult to use them as strain indicators.Therefore, we utilized mafic enclaves for the strain determination.Only the dioritic ellipsoidal enclaves were selected for strainestimation; ellipsoidal enclaves with pressure shadows were exclud-ed from this study. Thus, the rheological contrast is negligible andbreak-up of the enclaves during magma emplacement is unlikely(Tobisch and Williams, 1998).

Well-preserved enclaves occur throughout the Fangshan plutonand consist of 50–60% plagioclase, 10–15% amphibole, 8–12% biotiteand 13–32% other minerals (quartz and accessory minerals). Thus,there is very little rheological contrast between the enclaves and thehost rocks (Figs. 3 and 7). Using the ellipsoidal enclaves as strainmarkers is controversial because their original shapes were notspherical and they may have had large axial ratios. The final shapesand orientations of the enclaves are a function of complex magmaticprocesses. Enclaves are normally hosted by magma for certainperiods of time as relatively rigid objects that undergo rotation andmay break up (Paterson et al., 2004; Tobisch and Williams, 1998).However, it is possible to obtain some useful data from these

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Fig. 7. Photos of the ductile shear zone and brittle solid-state deformation in the northwestern part of the quartz monzodiorite unit of the Fangshan pluton. (A) Dextral strike-slipductile shear zone indicated by S–C fabric and deformed dark dioritic enclaves; (B) dextral strike-slip ductile zone indicated by S–C fabric; (C) Sinistral strike-slip ductile shear zoneindicated by deformed dark enclaves and S–C fabric. Note the Ф is defined by the angle between the X axis and the S foliation; (D) porphyroclastic structure shown by rotatedplagioclase and the S–C fabric shows sinistral strike-slip movement. Diameter of coin is 19 mm; (E) sinistral strike-slip fault and the relationship between the X axis, the S foliationand Ф; (F) mineral lineation defined by epidote and amphibole.

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features by reconstructing original parameters from careful fieldobservation (Tobisch and Williams, 1998). The measured strain dataare only semi-quantitative for primary magma strain and solid-statestrain.

For each ellipsoidal enclave axial values (X, Y and Z) and directionand plunge of the X axis were measured on Smag/joint surfaces(Fig. 3D–E). The Ф value was defined and measured as the maximumangle between the X axis and S foliation of the solid-state deformation(Fig. 7A, C and E).

A total of 328 datasets for the enclaves was collected (Appendix I).The ellipsoidal enclaves were grouped if they occurred in an area andhad similar X/Z ratios. A total of 22 such groups were recognized (G1–G22 in Table 3 and Fig. 8E), and for each group the average value of theX/Z ratios and the mean dip direction and dip angle were calculated.Almost all have correlation coefficients between X and Z higher than0.8 (except G10 with a correlation coefficient of 0.78), indicating thatonly a small percentage of the enclaves (b20%) are significantlydifferent from the others in axial ratios (Table 3).

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Fig. 8. A: Geological map of the northwest part of the Fangshan pluton. Map shows the conjugate ductile shear zone; B–D: the equal area projections (lower hemisphere) of the inferred fabrics, including S and C foliations, and minerallineations; E: the plots of the ratios of ellipsoidal enclave groups and isoline of Rftot (X/Z) in the Fangshan pluton. Note that the data of only Rftot (X/Z) are the result of a single enclave measured by Li (1987).

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Table 3The group average values, including X/Z and the dip direction and dip angle of X axis,and X–Z correlation coefficient of the groups.

Group Number ofmeasurement

Mean of X-axis AverageX/Z

Correlationcoefficientof X/Z

Part withinthe pluton

Dipdirection

Dipangle

G1 12 201 11 4.1:1 0.863 North-westernG2 22 191 11 4.7:1 0.887G3 26 203 11 5.2:1 0.832G4 21 185 10 4.8:1 0.964G5 16 171 13 4.7:1 0.923G6 16 173 15 5.4:1 0.825G7 16 172 11 7.0:1 0.813G8 13 173 11 7.7:1 0.891G9 17 167 11 8.0:1 0.874G10 11 129 25 3.3:1 0.781 South-easternG22 5 134 35 3.8:1 0.897G11 12 135 18 2.0:1 0.87 CentralG12 10 138 21 2.8:1 0.904G13 20 161 12 3.0:1 0.852G14 9 136 15 3.2:1 0.827G15 16 146 39 2.4:1 0.864G16 15 163 28 2.6:1 0.862G20 11 129 29 2.2:1 0.947G21 10 129 31 2.3:1 0.863G17 15 129 14 3.5:1 0.907 North-easternG18 18 154 14 3.0:1 0.872G19 17 144 21 4.8:1 0.953

115D.-P. Yan et al. / Lithos 123 (2011) 102–120

The average X/Z ratios of the 22 groups of the enclave populations,together with measurements by Li (1987), are plotted in the geologicalmap of the pluton (Fig. 8E), producing isolines of total strain (Rftot). Ingeneral, the isolines crosscut the monzodiorite–quartz monzodioritepetrological boundary at a small angle. A maximum X/Z ratio of 78.0 isrecorded in the northwestern part of the pluton (Li, 1987). Ratios higherthan4.0were recorded in the northwestern and southeasternparts, andratios less than 4.0 were recorded in a northeast-trending zone fromNinfengpo to Fenghuangting to Nanguan, where only magmatic fabricswere found. The plunge angles of the X-axes are generally less than 40°;the Y-axes plunge steeply to the northwest; and the Z-axes plungegently to the southeast (Fig. 8B).

All 328 sets of themeasured X, Y and Z values for the enclaves wereplotted on a Flinn diagram (Fig. 9A), and K values, defined as K=a/b=(1+e1)(1+e3)/(1+e2)2=XZ/Y2, were used to divide the en-clave populations into two fields. A field with K around 1 or KN1,which includes G11–17 and G19–21, is located along the northeast-striking belt through the center of the pluton where X/Z ratios are lowand only magmatic fabrics are present. A second group with Kb1 islocated in the northwestern and southeastern parts of the pluton,where the rocks have clear solid-state fabrics (Fig. 8E).

The solid-state fabrics, magmatic fabrics, X/Z ratio distributionsand plots in the Flinn diagram all imply that late, solid-statedeformation was pronounced in the northwestern part, weak in thesoutheastern part, and very weak in the central and northeasternparts. Thus, the long axis vs. short axis ratios of 1.5 to 3.5 (Figs. 8E and9B–C) in the central and northeastern parts might approximatelyrepresent themagmatic strain plus the primary elliptical shapes of theenclaves (Rfmag+Rp) when they were generated by magma mingling(Paterson et al., 2004; Tobisch and Williams, 1998). Because groupassemblages G1–3, G4–6 and G7–9 in the northwestern part (Figs. 8Eand 9B–C; Table 3) have similar means of X/Z ratios and a close spatialassociation, their Rf/Ф values are plotted in Fig. 9B–C. This plot showsthat Rs is mostly in the range of 3.5 to 4.0 and the sum of Rfmag and Rpranges from 3.0 to 3.5. The Rfmag+Rp values are consistent with theestimated ranges in the central and northeastern parts. In thesoutheastern part (Fig. 8E), there are not enough enclave populationsfor a detailed analysis because of strong weathering and poor outcrop.However, based on an Rftot of up to 6–7, the presence of a weakly

developed solid-state fabric and several single enclavemeasurements,the sum of Rfmag plus Rp is roughly estimated to be 3.0–3.5 and Rs is1.5–2.0 or less.

5. Discussion

5.1. Temporal relationships of the Fangshan plutonic rocks

The quartz monzodiorite was previously dated at 130.7±4.0 Mausing the SHRIMP zircon U–Pb method (Cai et al., 2005). Previousstudies of the monzodiorite yielded K–Ar biotite ages ranging from131.0±5.4 to 132.8±0.12 Ma (BBGMR and CUG, 1988), and 40Ar/39Ar biotite ages between 133.0±0.9 and 132.7±1.4 Ma (Liu andWu1987), suggesting that the quartz monzodiorite postdated themonzodiorite, consistent with the observed field relations. Thesedates are essentially consistent with our monzodiorite 40Ar/39Arhornblende ages of 134.3±0.4 Ma and 136±0.5 Ma (Table 1, Fig. 4),and document an Early Cretaceous magmatic event in the Fangshanarea. Quartz monzodiorite apophyses cropping out in the Nanjiao areahave crystallization ages of 133.0 Ma and 134.0 Ma (Zhang et al.,2008a, LA-ICP-MS zircon U–Pb ages) and are simultaneous with theFangshan pluton.

5.2. Origin of the adakitic magma of the Fangshan pluton

Similar values of 206Pb/204Pb of 16.6090 and 16.5902, 207Pb/204Pbof 15.1745 and 15.1795, and 208Pb/204Pb of 36.4767 and 36.2007 for K-feldspar within the monzodiorite and quartz monzodiorite, respec-tively (Cai et al., 2005), indicate a comagmatic origin for these twophases. Negative εNd(t) values of −14.2 to −13.6, low 143Nd/144Ndratios of 0.5118–0.5119, positive εSr(t) values of +14.4 to +15.5 andlow initial 87Sr/86Sr isotopic ratios of 0.7053–0.7055 of whole rocks(Cai et al., 2005) suggest that the melts were derived from either thebasaltic lower part of a thickened crust or lithospheric mantle (c.f.Rapp et al., 2002).

The Fangshanplutonhas geochemical characteristics (Table 2, Figs. 5and 6) resembling those of adakites (Defant andDrummond, 1990). Thedepletion of Y and HREE, enrichment of Sr and LREE, and absence of Euanomalies (Table 3, Figs. 5 and 6) all suggest that garnet was a residualphase in the source,with little ornoplagioclase (Defant andDrummond,1990; Defant and Kapezhinskas, 2001; Peacock et al., 1994). The strongnegativeNband Ti anomalies indicate that the sourcehad residual rutileand amphibole, because rutile is an important carrier of these high-fieldstrength elements during partialmelting of hydrous basalts (Foley et al.,2000; Schmidt et al., 2004), with Nb additionally hosted in hornblende(Pearce and Norry, 1979). Thus, we conclude that themagma source forthe Fangshan adakitic plutonwas most probably rutile-bearing eclogiteor garnet amphibolite.

It is widely accepted that adakitic magmas can be produced bypartial melting of subducted oceanic slabs (Defant and Drummond,1990; Moyen, 2009). Alternative models for the petrogenesis ofadakites include fractional crystallization of basaltic magma (Castilloet al., 1999) and partial melting of basaltic lower crust of a tectonicallythickened continental crust (Atherton and Petford, 1993; Kay and Kay,2002; Muir et al., 1995; Wareham et al., 1997). In a thickened crust,delaminated lower continental crust may be associated withasthenospheric upwelling and generation of adakitic magmas (Gaoet al., 2004; Rapp et al., 2002; Xu et al., 2002, 2006). Experimentalstudies suggest that partial melting of eclogite or garnet amphiboliteat pressures greater than 1.2 GPa, equivalent to a crustal thickness ofmore than 40 km can form adakitic magmas (Rapp andWatson, 1995;Rapp et al., 1999, 2002). During the Early Cretaceous, the Fangshanarea was located within the northern part of the North China Block,where it had an estimated crustal thickness of more than 50 km,significantly more than its present thickness of 35–40 km (Gao et al.,2005; Ren et al., 1990). The metamorphic core complexes in the

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Fig. 9. A:Flinndiagramsshowing strain typesof theellipsoidaldarkenclavegroups. Thevertical axis isRXY=X/Y=(1+e1)/(1+e2),whereas thehorizontal axis isRYZ=Y/Z=(1+e2)/(1+e3);B–C: Rf/Ф diagrams of enclaves from G1+G2+G3 and G4+G5+G6, of which the locations are shown in Fig. 5E. For the data plotting and strain estimation using the Rf/Φ method, Rs isestimatedusing50% curve,which corresponds to calculatedRs values and separates into2quadrantswith equivalent points (Yu andZheng, 1984).Rf=final strainused to indicate the observedenclave shape, Rs=estimated strain, which represents solid-state deformation, Rfmag=magmatic strain produced bymagma intrusion, and Rp=the original shape of the enclaves when theyentered the magma.

116 D.-P. Yan et al. / Lithos 123 (2010) 102–120

Yanshan region and extensional tectonic features leading to thinnedcrust have been dated at ~125 Ma (Davis et al., 2002), 5–10 m.y.younger than the emplacement of the Fangshan pluton. Davis (2003)has shown that adakitic magmatism ended and alkalic magmatismbegan at 130–120 Ma in the Yanshan belt coincident with NW–SEextensional tectonics throughout eastern Asia. Adakitic geochemicalaffinities of the Fangshan pluton are very similar to Cenozoic K-richadakites in the N–S-trending rift in the Tibetan plateau, which wereinterpreted to be produced by partial melting of amphibole-bearingeclogites in the lower part of the thickened Tibetan crust duringextensional collapse of the Tibetan plateau (e.g. Hou et al., 2004;Wang et al., 2005; Guo et al., 2007). Thus, we suggest that theFangshan adakitic pluton formed by partial melting of a thickenedcontinental crust at ~130 Ma and that the melting coincided with thebeginning of crustal thinning.

5.3. Ascent and emplacement of the Fangshan pluton

Although magmatic and solid-state fabrics in the Fangshan plutoncan overlap in age, solid-state fabrics were generally producedsubsequent to magmatic fabrics, because the crystallized mineralswere re-deformed and cut by S–C fabrics in the northwest part of thebody (Fig. 7). Therefore, the magmatic fabrics might represent theearlier stage of magma ascent, whereas the solid-state fabrics representslightly later stage magma emplacement (Tobisch andWilliams, 1998).

In the Fangshan pluton, a consistent magmatic lineation plungingsoutheast at a high angle (Figs. 1 and 2) indicates a simple, consistentflow regime with slightly oblique (but almost vertical) movement ofmagma fromSE toNW.This earlier ascendingmagmadevelopedslightlystrongermagmatic fabrics towards themargin of the pluton andoriginalenclave ratios Rfmag+Rp of 3.0–3.5 (Rfmag values are not separated from

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Rfmag+Rp), as indicated by injected dioritic enclaves (Figs. 8E and 9).These magmatic fabric features, integrated with high-temperatureshear aureole with pluton-side-up kinematic indicators and rimsyncline around the pluton (He et al., 2009), suggest that a diapiricprocess (Berner et al., 1972; Cruden, 1988; Marsh, 1982; Paterson andVernon,1995; Ramsay, 1989;Weinberg andPodladchikov, 1994) for themagmatic ascent in this stage, as proposed by Shan et al. (2004) and Heet al. (2005, 2009), is appropriate.

However, the pluton also has asymmetric solid-state fabrics showingintense deformation in the northwestern part, weak deformation in thesoutheastern part, and little or no deformation in the northeastern,southwestern and central parts of the body. These fabrics areaccompanied by an increase in solid-state strain Rs of 1.5–2.0 to 3.5–4.0from the southeastern to northwestern parts of the pluton, as indicatedby the Rs of dioritic enclaves (Figs. 8E and 9). These features suggest thatthe deformation was stronger towards the northwest, probably causedby later-stage oblique magma emplacement (Figs. 1, 2, and 8A and E).

The Fangshan pluton and Nanjiao apophyses are similar in terms oftheir ages and compositions, suggesting similar origin. The arcuate Beilingsyncline, which is located between the Fangshan pluton and Nanjiaoapophyses and is superimposed on a pre-existing east–west-strikingsyncline, was interpreted to have been produced by the obliqueemplacement of the Fangshan pluton (Fig. 1B–C; Yan et al., 2006; Zhanget al., 2008a). By analyzing the stratigraphic relationship and deforma-tional sequence, a composite detachment fault system, which producedthe D1 low-angle normal fault and D2 and D3 thrust faults, has beeninferred beneath the Beiling syncline (Shan et al., 1991; Yan et al., 2006).The existence of plutonic rocks at depth is inferred from magneticanomalies of more than 200 nT in the region (GGTBBGMR, 1987).Aeromagnetic polarization with upward continuation to 500 m showsincreased magnetic anomalies from 200 nT to 700 nT from Nanjiao toFangshan (Fig. 10A), whereas aeromagnetic polarization with upwardcontinuation to 1500 m has less than 200 nT magnetic anomaly in theNanjiao area (Fig. 10B), indicating a southeast-dipping plutonic body(GGTBBGMR, 1987) (Fig. 1C). This obliquemagmatic body coincideswiththe inferred southeastward-dipping detachment fault system (Shan et al.,1991; Yan et al., 2006). Using muscovite 40Ar/39Ar dating, Wang and Li(2007) obtained the age of the ductile deformation associated with thereactivation of the detachment fault in the Nanjiao area at 133.0 Ma,

Fig. 10. Contour diagrams of aeromagnetic polarization (A) upward continuation to 500 m, anvariable depth.

which is simultaneous with the crystallization of Fangshan and Nanjiaoplutons (Zhang et al., 2008a). Therefore, we suggest that this pre-existingdetachment fault system underneath the Beiling syncline probably actedas a preferred pathway for lateral migration and emplacement of themagma(Figs. 1 and11). This subsequentmagmatic emplacementnotonlydeformed thewallrock of the Beiling syncline (wallrock strain) by diapiricintrusion, but also re-activated the existing detachment fault system(fault-related space formation) (Fig. 11).

Based on the above discussion, we conclude that the Fangshan plutonrecorded two stages of magma intrusion (Fig. 11). In the earlier stage,adakitic magmas produced by partial melting of the thickened crustascended quickly through the thinning middle crust by diapirism(perhaps also with ballooning) (He et al., 2005, 2009). During magmaascent, tabular or acicular crystals of amphibole, biotite and plagioclasewere aligned parallel to the flow direction, and thus formed the observedmagmatic fabrics with steep dips. The mingling of mafic and/orintermediate magmas with the felsic host magma produced maficenclaves (Fig. 3F), and generally small primary strain (Rp) (John andBlundy, 1993; Tobisch andWilliams, 1998; Vernon, 1983, 1988;Williamsand Tobisch, 1994). Continuous shearing produced during differentialflow of the host magma typically modified the enclaves into ellipsoidalshapes with the strain of Rfmag (Tobisch andWilliams, 1998). This meansthat there was an internal pressure in the magma chamber against thewallrock (Fig. 11).

At a later stage, intrusion of the younger quartzmonzodioritemagmaproduced the observed solid-state deformation in the marginal beltcontemporaneous with cooling. Development of the Beiling synclineimplies reactivation of the pre-existing detachment fault systembeneaththe syncline, which might produced a pathway for magma migration(Yan et al., 2006; Zhang et al., 2008a). Therefore, the obliqueemplacement of this magma was, at least partly, related to spaceproduced by faulting (Figs. 1 and 11). However, deformation of theBeiling syncline by the injected magma suggests that diapirism was alsostill active at this time.

5.4. Implications for crustal thinning of the East China Plateau

Increasing number of adakitic plutons associated with extensiveMiddle Jurassic to Cretaceousmagmatismhave been recently recognized

d (B) upward continuation to 1500 m, which indicate magnetic anomaly of the rocks in

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Fig. 11. A sketch diagram showing an emplacement history that involved earlier diapiric and later both diapiric and fault-related processes for the Fangshan pluton.

118 D.-P. Yan et al. / Lithos 123 (2010) 102–120

in the eastern part of the North China Block (Cai et al., 2005; Davis, 2003;Deng et al., 2004a, 2004b; Xu et al., 2006; Zhang et al., 2008b) (Fig. 1A)and thewhole of the East China Block.Manyof these appear to have beengenerated by melting of the lower crust (Davis, 2003; Gao et al., 2004;Wang et al., 2006; Xu et al., 2002; Zhang et al., 2008b, 2001). Thickeningof the East China Block is believed to have been produced by subductionof the South China Block beneath the North China Block during theTriassic (Liou et al., 1996; Yin and Nie, 1996; Yan et al., 2006). Thewidespread occurrence of adakitic plutons generated by partial meltingof the lower crust suggests an original crustal thickness of N40–50 km,which in turn defines the East China Plateau (Zhang et al., 2001, 2008b).The plateau appears to have been produced by the subduction of theSouth China block beneath the North China block during Triassic time(Yin and Nie, 1996; Yan et al., 2006). The Fangshan pluton (D4) iscontemporaneous with the large-scale Cretaceous magmatic activitywithin the Yanshan and Taihang Mountains (Fig. 1A) and eastern China(Davis, 2003; Deng et al., 2004a, 2004b; Wang et al., 2006). The residualmaterials after extraction of the adakitic magmas were garnet-bearingeclogite or garnet amphibolite and thus,were denser than theunderlyinglithosphericmantle. This could have lead todelamination and thinning ofthe lower crust (Gao et al., 2004), and upwelling of the asthenosphere.This process is also consistent with the development in the North ChinaBlock of numerous Early Cretaceousmetamorphic core complexes (Daviset al., 1996, 2002; Darby et al., 2001) and simultaneous (Late Mesozoic)extensional basins (Meng et al., 2003; Ren et al., 2002). Temporal andspatial relations suggest that these extensional structureswere formed inresponse to the collapse of a thickened crust beneath the East ChinaPlateau (Davis et al., 1996, 2002;Renet al., 1990, 2002;Menget al., 2003).The change from diapiric magma ascent to a combined mechanism ofdiapiric and fault-related emplacement in the Fangshan pluton provides

important constraints on the thinning and extensional structures in eastChina during the Cretaceous.

6. Conclusions

The Fangshan pluton was emplaced along a pre-existing detach-ment fault at 136.0–131.0 Ma. This Cretaceous pluton has adakiticaffinities and was formed by partial melting of the lower part of athickened continental crust in the East China Plateau. Magmaticfabrics, the strain of ellipsoidal dioritic enclaves and solid-statedeformation suggest that the history of magma emplacementinvolved originally diapiric process followed by a combination ofdiapirism and fault-generated space. The generation of the adakiticmagmas and their subsequent emplacement most likely coincidedwith the advent of crustal thinning of the East China Plateau duringthe Cretaceous.

Acknowledgments

This study was supported by the Natural Science Foundation ofChina (40921062, 40872140), National Basic Research Program ofChina (2009CB421001) and the 111 Project (B07011). We havebenefited from helpful discussions with Profs. Zhaoren Fu andChanghou Zhang during the study. Thanks are due to Ms. Xiao Fu,Dr. Weihua Sun and Prof. Yu Wang for laboratory work. Enlighteningdiscussions and assistance throughout this study by Prof. P.T.Robinson and reading an earlier draft of this paper by severalcolleagues, and constructive comments and suggestions on themanuscript by two anonymous reviewers and Editors are very muchappreciated.

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Appendix A. Supplementary data

Supplementary data to this article can be found online atdoi:10.1016/j.lithos.2010.11.015.

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