origin, ascent and oblique emplacement of magmas in a ...· origin, ascent and oblique emplacement
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Lithos 123 (2010) 102120
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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@example.com (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(10671348 ppm) and low Y (817 ppm) with high Sr/Y ratios (71137). 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 3.03.5. Asymmetric solid-state deformational fabrics and strains (Rs) for the northwestern (3.54.0) and southeastern parts (1.52.0)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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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.,
103D.-P. Yan et al. / Lithos 123 (2011) 102120
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 QinlingDabie orogenic beltto the south, and by the MongolOkhotsk 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 JurassicCretaceous 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 QinlingDabie 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 TanLu 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 northsouthcrustal 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 YanshanYinshan 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 theYanshanYinshan 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)e