Fig. 3. Photomicrographs of garnet (a), Cr-spinel (b) and Mica (c) xenocrysts in Ogonek pipe; SEM Image of two variety of K-feldspar in volcanic breccias (d-f)

Fig. 4. a,c,d - volcanic clasts (autoliths) samples of Perevalnaya pipe , b -radiaxial feldspar aggregate from Gornaya pipe (cross nicols); d,e- inequigranular medium- to fine-clastic volcanic breccias (cross nicols); f - flow texture, Sample intrusion 104.

Mantle xenocrysts of Chompolo field of the alkaline volcanics, Aldan shield, South Yakutia

E. Nikolenko, N. Tychkov, V. Afanasiev V.S. Sobolev Institute of Geology and Mineralogy SB RAS, Novosibirsk, Russian Federation

Fig. 10 The compositional range of the Cpx in shown in the triangular composition diagram. Fig. 5. Contamination index vs the Ilmenite index.The fields of Group I, Group II

kimberlites and lamproites, are shown for comparison (Chalapathi Rao et al., 2003) (a); TiO2 vs K2O for group I and II kimberlites (after Smith et al, 1985) (b).

Fig.11. Cr-Diopsides from Chompolo pipes in modified Ramsay's (1992) classification diagram. Solid line - modified boundary between Gar-lherzolites and Sp-lherzolites.

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Fig. 12. PT diagrams for mantle rocks from the Chompolo alkaline rocks after thermobarometer (Nimis and Taylor, 2000);The boundaries of phase transitions: G/D – graphite/diamond (Kennedy and Kennedy, 1976)

The lithospheric thickness (pressure) was estimated using a P38 monomineral garnet barometer (Grutter et al., 2006), for harzburgitic and lherzolitic pyropes. The barometry is originally designed for the cases of pyrope-chromite coexistence, i.e., for Cr-saturated conditions. If this coexistence is questionable, the use of a monomineral barometer may be possible as well, but the results will

Introduction The Chompolo field of alkali volcanics was discovered in 1957-58 by geologists of the Amakinsky Survey Group. The breccia dike and pipe structures in the field, composed of wallrock xenoliths, contained typical kimberlitic minerals (pyrope, Cr spinel, and Cr diopside) and were classified as kimberlites (Shilina and Zeitlin,1959). More detailed diamond exploration in the area in 1959-1960 was discouraging but later the area was reconsidered as potentially diamondiferous after twenty four (-1 mm) diamond crystals were found in 1988-1986 (Mudrik, 1988). The Chompolo field belongs to the Aldan lamproite province where such igneous rocks are widespread and easily accessible Fig. 2 (Tobuk-Khatystyr, Lower & Upper Yakokut fields, Murun and Lomam plutons) and have been described in numerous publications (Vladykin, et al., 1991; Panina, 1993; Panina and Vladykin, 1994; Vavilov et al., 1986; Davies et al., 2006). Two events of alkaline magmatism within the Aldan province, in the Early Proterozoic and Mesozoic (T3-K2) produced, respectively, the Chara-Aldan and Lena-Aldan subrpovinces (Bogatikov et al., 1991). The igneous rocks were interpreted previously either as kimberlites (Shilina and Zeitlin, 1959; Utrobin, 1990; Kostrovitsky and Garanin, 1992; Mironyuk, 1998; Aschepkov et al., 2001), or as lamprophyres and lamproites (Vladykin,1985; Bogatikov, 1991; Panina, 1993; Panina and Vladykin,1994;Kornilova, 1997; Davies et al., 2006). The rocks of the Chompolo field are hard to access for sampling and remain poorly studied and little reported. In this study we provide new mineralogical, petrographic, and chemical data on the Chompolo alkalic igneous rocks

Geological backgroundThe Aldan lamproite province formed in a collisional setting upon a Precambrian cratonic basement (Bogatikov et al, 1991) The Chompolo area belongs to the Central Aldan superterrane of the Aldan-Stanovik shield and lies at the Amga junction between the West Aldan granite-greenstone and the Nimnyr granulite-orthogneiss terranes. The present-day erosion surface exposes a series of closely spaced N—S faults that separate fragments of Archean and Early Proterozoic granite-greenstone and granulite-orthogneiss belts. The faults accommodate dikes of dolerite and alkaline rocks, kimberlite-like bodies, and large intrusions bearing alkali-earth elements (Fig. 1). The sedments consist of dolomite and limestone intercalated with Early Cambrian and Late Proterozoic conglomerate. Geological constraints from indicator minerals found in Lower Jurassisediments (Vladimirov et. al., 1989) place the age of the Chompolo kimberlitesand kimberlite-like rocks at pre-Jurassic, and Bogatikov et al. (1991) suggest a post-Early Triassic age of the kimberlite field. Isotopic age determinations (Rb-Sr isochron method) of the kimberlite-like body called Intrusion 104 indicate a younger age of 131±4 Ma (Zaitsev and Smelov, 2010). .

Sampling and analytical techniquesThere are ten dike, pipe, and vein structures within the Chompolo field, varying in size from 800 x 25 m (Aldan dike) to 150 x 80 m (Ogonek pipe) or 100 x 40 m (Gornaya pipe) Rock and mineral amples were collected from six kimberlite-like structures (Aldan, Sputnik, Gornaya, Ogonek, Perevalnaya, and Kilier-E)during field trips of 2012 and 2013. The analytical work was carried out at the Institute of Geology and Mineralogy (Novosibirsk). Mineral chemistry was studied on a JEOL JXA-8100 electron microprobe . Rare earths were determined on a Finnigan MAT ELEMENT high-precision ICP mass spectrometer with a U-5000AT+ ultrasonic nebulizer, using the method of Li metaborate fusion ICP-MS (Nikolaeva et al., 2008). Major oxides were measured by X-ray fluorescence (XRF). The samples were examined on a TESCAN MIRA 3 LMU scanning electron microscope with an EDS Inca Energy 450+ detector.

Samples of the Gornaya, Sputnik, Perevalnaya, and Ogonek structures of the Chompolo field are inequigranular medium- to fine-clastic volcanic breccias with flow texture and a fine-grained groundmass (Fig. 3 and 4). The percentages of wall rock clasts and autoliths range from 20 to 60 %. They are mostly quartzite (micro-quartzite), feldspar and quartz-feldspar, or sedimentary silicic rocks with variable amounts of mica minerals (Fig. 3). The quartz grains are xenogenic and have diverse clastic shapes, from isometric to flat (Fig. 4). Clasts of volcanic rocks are less abundant and enclose high percentages of feldspar laths and different amounts of opaque minerals. The volcanic lithoclasts are similar to the breccia groundmass in phase composition, structure, and texture, and are thus interpreted as autoliths. The fine cryptocrystalline groundmass consists of chlorite, K-feldspar, minor opaque minerals, and mica (muscovite and biotite). The presence of mica in the groundmass may be due to abundant clasts of sedimentary or high-silica igneous rocks. K-feldspar and plagioclase of another variety occur as angular clasts and often contain BaO impurity (up to 2.7 wt. %). This variety appears to be more stable against secondary alteration, which may be evidence for its xenogenic origin. All samples contain layered minerals (mica-vermiculate, vermiculate, smectite, kaolinite, and chlorite) derived from feldspar, mica, etc. Phenocrysts include chromites (Fig. 3), as well as sporadic Sr apatites (up to 2.1 wt.% SrO), rutiles, and zircons, and few garnets (Fig. 3) coated with secondary mineral aggregates (chlorite and carbonate). XRD analysis has revealed the presence of pyroxene (diopside), amphibole, Sr- and Mn-bearing dolomite, siderite, and calcite. The rock contains abundant secondary reddish-brownish aggregates as patches or henocrysts, possibly formed by substitution of mica minerals. . Thus, the igneous bodies in the Chompolo field (except Intrusion 104) are inequigranular volcanic breccias with micro- or crypto-crystalline groundmass of K-feldspar (up to 16.3 wt.% K2O, up to 3.2 wt.% FeO), chlorite, opaque minerals, melanocratic phenocrysts (garnet, pyroxene, amphibole, Cr spinel, apatite, zircon, mica), and abundant wallrock and basement clasts.

Minerals Chemistry Garnet (pyrope) chemistry records the presence of mantle and crustal material. Mantle mineralization prevails in the Aldan dike and the Sputnik, Gornaya, and Ogonek pipes, while crustal and eclogitic signatures are found in the Perevalnaya and Kilier-E pipes. Mantle garnets lack the common megacryst, wehrlite, and high-temperature lherzolite varieties (Fig. 8). Dunite-harzburgite garnets are always present in minor amounts, without subcalcic high-Cr varieties. Most of mantle garnets are of lherzolitic paragenesis. Depleted lherzolite garnets are more often of shallow than deep origin. Nevertheless, there are up to 10 mol.% of deep-seated lherzolite garnets with the knorringite endmember. Crustal garnets are most often of deep granulite origin. Cr spinel (1064 analyses) occurs as 0.2-2.0 mm macrocrysts extracted fromthe heavy fraction of the samples. They are sometimes well faceted octahedrons, most often ith strongly rounded corners and edges, or segregations of different shapes filling the interstitial space.

Monomineral Cpx thermobarometry (Nimis 2001) was performed on Cr-diopsides with more than 0.5 wt.% Cr2O3 typical of alkaline igneous rocks. When selecting the data for thermobarometry, several compositional groups of diopsides were excluded. They were, namely (I) non-peridotitic clinopyroxenes; (II) clinopyroxenes from spinel peridotite (Fig.11); (III) Cr diopsides with Al+Cr<Na +K and (IV) those with(Al+Cr) - (Na+K) = from 0 to 0.01. The latter group was excluded to avoid pressure overestimation known to occur at the (Al+Cr) - (Na+K) difference below 0.01; therefore, correct thermobarometry can be achieved at (Al+Cr) - (Na+K) > 0.01 in Cr diopsides. As a result, only 300 out of more than 1800 analyses remained for the thermobarometry study. . The obtained Cpx thermobarometry results indicate a lithospheric thickness in the sampled Chompolo field no less than 130 km (4.1 GPa) (Fig.12a). The temperature and pressure points plot along the 35 and 40 mW/m2 geotherms. Igneous bodies in the Chompolo field show alsmot identical geotherm patterns with a characteristic kink in the high-pressure region, which is evidence of interaction between depleted lithosphere and hotter enriched asthenospheric melts. This interaction is presumed to occur commonly within the lithosphere-asthenosphere transition. A heat flow of 35 mW/m2 is typical of cratonic lithosphere at least 2.5 Ga old and up to 300 km thick. In the case of samples we studied, heat flow disagrees with the maximum lithospheric depth of 130 km, though the age of the crust (indicative also of the lithospheric age) is older than 2.5 Ga in the Aldan shield.

Single clinopyroxene thermobarometer

ConclusionsWe infer that the Chompolo alkali volcanic rocks are not kimberlites but rather low-Ti alkaline rocks similar to those in the Aldan province. Unlike the Aldan lamproite(Tobuk-Khatystyr field, tc.), the Chompolo rocks contain both crustal and mantle minerals. The compositions of pyrope and Cr spinel, as well as thermobarmetry estimates of lithospheric thickness at the time of magmatic activity, indicate that the Chompolo rocks are diamond-barren. RFBR 15-05-04885.