CONDITIONS OF FORMATION OF GABBRO-PERIDOTITES SILLS OF THE DOVYRENSKY INTRUSIVE COMPLEX (NORTHERN BAIKAL REGION)

Mineralogical and geochemical study of the gabbro-peridotite sills located in the near-bottom part of the Yoko-Dovyren stratified massif among the host terrigenous-carbonate rocks showed that the thickest (200–250 m) of them are differentiated from plagiolherzolites to olivine gabbronorites. Their formation is well described by fractional crystallization of the picrobasalt melt. When it entered the crystallization chamber, it had already contained some intratelluric crystals of high-magnesian olivine (up to 93 % Fo). The rock crystallization occurred in the temperature range 1234–985 °C at pressure 1.3–1.6 kb, corresponding to depth ~5–6 km. The calculated depth of separation of the initial melt from the mantle source was ~88 km, which corresponds to the lithostatic pressure ~28 kbar. In terms of geochemical parameters, the composition of the initial melt is characterized by dual nature: they are close to both the basalts of suprasubduction magmatism and the basalts of collisional volcanic-plutonic areas. We assume that formation of the Synnyr rift with the Dovyren intrusive complex is due to the destructive development of scattered spreading zones at the collisional-accretionary stage of the Baikal-Muya belt within 720–800 Ma

УСЛОВИЯ ФОРМИРОВАНИЯ СИЛЛОВ ГАББРО-ПЕРИДОТИТОВ ДОВЫРЕНСКОГО ИНТРУЗИВНОГО КОМПЛЕКСА (СЕВЕРНОЕ ПРИБАЙКАЛЬЕ)

Mineralogical and geochemical study of the gabbro-peridotite sills located in the near-bottom part of the Yoko-Dovyren stratified massif among the host terrigenous-carbonate rocks showed that the thickest (200–250 m) of them are differentiated from plagiolherzolites to olivine gabbronorites. Their formation is well described by fractional crystallization of the picrobasalt melt. When it entered the crystallization chamber, it had already contained some intratelluric crystals of high-magnesian olivine (up to 93 % Fo). The rock crystallization occurred in the temperature range 1234–985 °C at pressure 1.3–1.6 kb, corresponding to depth ~5–6 km. The calculated depth of separation of the initial melt from the mantle source was ~88 km, which corresponds to the lithostatic pressure ~28 kbar. In terms of geochemical parameters, the composition of the initial melt is characterized by dual nature: they are close to both the basalts of suprasubduction magmatism and the basalts of collisional volcanic-plutonic areas. We assume that formation of the Synnyr rift with the Dovyren intrusive complex is due to the destructive development of scattered spreading zones at the collisional-accretionary stage of the Baikal-Muya belt within 720–800 Ma.Минералого-геохимическое изучение габбро-перидотитовых силлов, расположенных в приподошвенной части Йоко-Довыренского расслоенного массива среди вмещающих терригенно-карбонатных пород, показало, что наиболее мощные (200–250 м) из них дифференцированы от плагиолерцолитов до оливиновых габбро-норитов и их формирование хорошо описывается фракционной кристаллизацией пикробазальтового расплава. При поступлении в камеру кристаллизации он уже содержал некоторое количество интрателлурических кристаллов высокомагнезиального оливина (до 93 % Fo). Кристаллизация пород происходила в интервале температур 1234–985 °С при давлении 1.3–1.6 кбар, отвечающем глубине ~5–6 км. Рассчитанная глубина отделения исходного расплава от мантийного источника составила ~88 км, что отвечает литостатическому давлению ~28 кбар. По геохимическим параметрам состав исходного расплава характеризуется двойственной природой: он близок как базальтам надсубдукционного магматизма, так и базальтам коллизионных вулканоплутонических ареалов. Можно предположить, что формирование Сыннырского рифта с довыренским интрузивным комплексом связано с деструктивными процессами развития рассеянных спрединговых зон на коллизионно-аккреционном этапе развития Байкало-Муйского пояса на рубеже 720–800 млн лет

The Val gabbro plutonic suite, Kerguelen Archipelago: Evolution of a volcanic feeder system in an oceanic island New (U-Th)/He age constraints on the emplacement of kimberlite pipes in north-eastern Kansas Isotopic geochemistry of the Arsentyev gabbro-sye

The petrology, composition and Sr-Nd-Pb-Hf isotopic geochemistry of basic and felsic rocks from the Val gabbro plutonic suite on the Kerguelen Archipelago constrain differentiation processes in sub-volcanic magma chambers and the role of variable magma flux rates in the formation of a major oceanic island. The 4 km 2 Val gabbro was forcefully emplaced at 24.25 ± 0.15 Ma (U-Pb zircon) into volcanic rocks of the Lower Miocene series on the Southeast Province. Cumulate gabbroic rocks are the dominant lithology, with horizontally layered olivine-and clinopyroxene-rich peridotitic cumulates at the base, overlain by coarse-grained olivine and/or clinopyroxene-rich gabbros and vertically layered finer grained equigranular gabbros. The Val gabbro was formed by repeated injections of crystal-rich and crystalpoor magmas into a magma reservoir where the main differentiation process was the segregation of earlier crystallized, dense mafic crystals. Striking geochemical similarities between the fine-grained intrusive rocks from the Val gabbro and the mildly alkalic basalts of the Lower Miocene series indicate that they were all derived from similar alkalic basaltic parental magmas that reflect the enriched Kerguelen mantle plume component. Prior to 25 Ma, magma flux rates associated with formation of the transitional flood basalts on the Kerguelen Archipelago were sufficiently high to prevent the establishment of sub-volcanic magma chambers capable of undergoing significant differentiation. At 25 Ma, the change to mildly alkalic basaltic volcanism was associated with deeper melting (garnet peridotite), lower extents of melting, and a lower total magma flux rate, which led to the formation and stabilization of high-level intrusions such as the Val gabbro plutonic suite. Twelve kimberlite pipes, identified by geological and geophysical exploration, are located above the mid-continent rift system in Riley and Marshall Counties, Kansas. Previous geochronological work on these rocks has yielded inconclusive estimates for their age. This study presents new data about kimberlite emplacement suing (U-Th)/He dating of apatite, titanite, and zircon from the Stockdale, and Tuttle kimberlites. Zircon from the Tuttle pipe and titanite from the Stockdale pipe give cooling ages of 105±10Ma and 101±15Ma respectively. These data are consistent with our Rb-Sr analysis of phlogopite megacrysts that give a 5 point isochron age of 106.5±3.8Ma. This demonstrates that (U-Th)/He thermochronometry provides reliable timing constraints on the cooling of common xenocrystic phases. Apatite (U-Th)/He ages range from 67.3±5Ma for the Stockdale pipe to 61.2±8Ma for the Leonardville pipe, suggesting a thermal pulse in latest Cretaceous to earliest Tertiary time. Fuid-inclusion data from calcite-magnetite veins cross-cutting the kimberlites records fluid temperatures of ~150-200°C (above apatite, but below zircon/titanite closure). Apatite (U-Th)/He analyses from nearby sandstones give Jurassic cooling ages, demonstrating that there was no regional thermal event after kimberlite emplacement. These observations, coupled with (U-Th)/He and Rb-Sr age data, suggest that the kimberlites were emplaced at ~105 Ma and subjected to local reheating by fluids at ~65 Ma. The Arsentyev massif is confined to the intrusions of syenite-pyroxenite-gabbro formation of high titanium ultramafic-mafic association. These intrusions are related to rift-like structures of various ages and close to alkaline basalts by geochemical characteristics. The two intrusive phases, each of them being followed by formation of the dike complex rocks, form the Arsentyev massif (Badmatsyrenova et al., 2004). The first phase consists of the stratified series of pyroxenite, olivine and kersutite gabbros, gabbros, anorthosites and syenite. The second phase includes the rocks of alkali-feldspar syenite series. The сoarse-grained alkaline syenites of II phase have resemblance to similar rocks Bichursky complex MZ. New (U-Th)/He age constraints on the emplacement of kimberlite pipes in north-eastern Kansas Isotopic composition of gabbro and syenite of I phase were studied. values, identical to or slightly lower than that of the associated mafic and syenitic rocks. They are differeciates from melts directly derived from the enriched mantle or from the newly formed mafic lower crust. The second group includes granitoids located in so-called "Shi-Hang" zone The two groups of high ε Nd (T) granitoids are both 150 to 160 Ma in age based on zircon SHRIMP U-Pb, Rb-Sr and 40 Ar-39 Ar dating. All these high ε Nd (T) granitoids reflect an extentional tectonic environment and represent an addition of the mantle derived meterials to the crust of the southeatern China at the late Jurassic. Distribution of these rocks along northeast striking faults suggests that Circum-Pacific tectonics was dominant by that time. References These results, combined with the bulk C-isotope values of the CM (δ 13 C -32.1 to -38.2‰ PDB, n=11), are suggestive of a biogenic source, and as such, may represent remnants of some of the earliest primitive life forms on Earth. Understanding the lipid biochemistry and carbon isotope fractionation of deep-sea piezophilic bacteria is of paramount importance because lipids and stable carbon isotopes are used extensively in marine biogeochemistry for defining the sources of organic matter and assessing preservation of organic matter in ocean sediments. However, our current understanding of microbial lipid biochemistry and carbon isotope fractionation is based on studies on lipid biosynthesis of surface bacteria at warm temperature and atmospheric pressure. The models and parameters of carbon isotope fractionation derived from surface bacteria may be significantly different from that of deep-sea piezophilic bacteria. In this study, we investigated carbon isotope fractionation in a piezophilic bacterium Moritella japonica DSK1. DSK1 was grown in natural seawater on glucose at atmospheric pressure (atm), 100, 200, and 500 atm. Phospholipid fatty acids were extracted and identified by GC-MS. δ 13 C of individual fatty acids was determined by a GC-C-IRMS. References Fatty acids detected include C 12-20 saurated, monounsaturated and polyunsaturated fatty acuds. The fatty acids exhibited distinctly different isotopic composition. Fatty acids from cells grown at higher pressure were considerably depleted in 13 C. For example, fatty acids from cells grown at 100 atm displayed remarkably less negative δ 13 C values (8.4 to 23.2‰), whereas fatty acids of cells grown at 200 and 500 atm showed much less variations (0.2 to 4.8‰). Our results suggest that hydrostatic pressure is an important factor in influening carbon isotope fractionation in marine bacteria and that caution must be exercised in using δ 13 C of fatty acids to determine the soiurce of organic matter in marine environment. The Late Jurassic Morrison Formation (famuous for its diverse dinosaur fossils and as a source of uranium ore), underlies a large area of the western interior of the United States. It was deposited deposited in a variety of terrestrial and lacustrine environments east of a magmatic arc along the western boundary of North America. As a result, much of the formation (particularly the Brushy Basin Member on the Colorado Plateau) consists of pyroclastic fall deposits erupted from volcanoes to the west of its outcrop belt. As such, the Morrison Formation provides an important record of Mesozoic magmatic activity along the western North American plate boundary. However, the exact locations of the pyroclastic eruptions are debated, and thus are not able to constrain tectono-magmatic models during the Mesozoic. In an effort to constrain the eruptive sources of pyroclastic deposits, we have analyzed Pb isotope ratios of sanidine separated from three samples of the Morrison Formation. To check for homogeneity and reproducibility, several subsamples of different weights were analyzed. In an individual sample, almost all of the ratios overlap at the 2-sigma error leveleven for samples as small as 10 mg (10s of grains). When all three Pb isotope ratios are considered, each sample is uniform, but distinct from the other samples. This is consistent with a lack of xenocrysts or detrital contaminants in the ash. The Pb isotope compositions of two of the beds plot in Zartman's (1974) lead isotope province 2 and one lies very close to the boundary between provinces 1 and 2. There are only a few plutons in these provinces (in southern Arizona-New Mexico and in southern California) that are currently known to have Late Jurassic ages. Consequently, future research should focus on magmatic centers in these provinces as potential eruptive sources for pyroclastic beds in the Morrison Formation, and tectono-magmatic models for the Late Jurassic need to include a period of significant magmatic activity in southern California and western Arizona during the Mesozoic. It is generally accepted that there is a structural control on trace element incorporation into crystals. By analogy, isotopic fractionation between fluid and a growing crystal surface may also occur. With the aim of examining the processes that govern mineral-fluid oxygen isotope fractionation at low to medium temperatures (150 to 350°C) in natural systems, variations in trace element and oxygen isotope compositions across and along different growth zones and sectors are being investigated in natural quartz crystals formed in veins during retrograde regional Alpine metamorphism. δ 18 O values measured for different parts of single crystals (center and rims; bottom and top parts) during a preliminary study were found to differ by close to 3‰ (values between 3.6 and 6.2‰). Similarly, Onasch and Vennemann (1995) have measured differences in δ Trace element and oxygen isotope zonations in growth sectors of natural quartz crystals 18 O values of up to 2‰ in different sectors of the same growth zone in sector-zoned quartz crystals, opening up the possibility of fluid-mineral disequilibrium partitioning of oxygen isotopes. Growth and sector zoning in the Alpine quartz crystals studied are highlighted using cathodoluminescence. Several profiles, selected on the basis of their cathodoluminescent appearance, had significant variations in Al, Fe, and Mg, detected using an electron microprobe. Most notable are changes in Al content that reach 100's ppm, with maxima of about 500 ppm in distinct growth zones. LA-ICPMS analyses indicated locally high concentrations of K, Ca, Ge, Li, Ti, Zr in some crystals but these anomalies could not be correlated to distinct growth zones. Additional analyses of oxygen isotope and trace element (Li, Ge, K,…) variations are planned using an ion microprobe as well as UV-Laser based in-situ measurements, in order to augment the data base and ultimately obtain information on growth mechanisms and their possible controls on elemental and isotopic fractionation processes for natural crystals. The Liaohe Group is an important Paleoproterozoic stratigraphic unit in the northeastern part of the North China Craton and traditionally is subdivided into the North and South Liaohe Groups. Associated with both the North and South Liaohe Groups are voluminous Paleoproterozoic granitoid rocks, named the Liaoji granitoids. Different tectonic models, including terrane amalgamation, continent-arc collision and rift closure, have been proposed to interpret the tectonic setting and evolution of the North and South Liaohe Groups and associated Liaoji granitoids. At the centre of the controversy between these models is whether or not the North and South Liaohe Groups developed on the same Archean basement. Nd isotopic geochemistry of the Liaoji granitoids provides important constraints on this controversial issue. The Liaoji granitoids associated with the North and South Liaohe Groups display similar εNd values, restricted to a narrow range from 0 to 2, implying that these granitoid rocks were derived from the same or a similar magma source. Moreover, the Liaoji granitoids associated with the North and South Liaohe Groups have similar Nd model age (TDM), ranging from 2.4 Ga to 2.6 Ga, suggesting that the protoliths of the Liaoji granitoids associated with both the groups may have formed simultaneously, and that the basement rocks underneath the Liaoji granitoids and associated North and South Liaohe Groups belong to the same continental block rather than two different blocks. Combining lithological, structural and geochronological considerations, we interpret the North and South Liaohe Groups as having developed on a single late Archean basement that underwent Paleoproterozoic rifting associated with the intrusion of the Liaoji granitoids and the formation of the Liaohe Group, and closed upon itself in the Paleoproterozoic. Reference Acknowledgement This research was financially supported by NSFC grants (40472098 and 40002015) We present the first combined trace element, Hf-Nd isotopic study from the marginal zone of the 1.27 Ga Muskox intrusion in order to constrain the extent and character of crustal contamination and the potential for formation of NiCu-PGE sulphide mineralization. The two regions studied, West Pyrrhotite Lake (WPL) and Far West Margin (FWM), have different characteristic chemical and isotopic variations. The WPL region is adjacent to metaplutonic rocks, is unmineralized, and records the complete stratigraphy of the marginal zone. The FWM region however is adjacent to metasedimentary rocks, is weakly mineralized, and the contact norite is not present. Trace element and isotopic data record a sharp increase in crustal contamination within 5 m of the contact in both regions, however elevated La/Sm and La/Nb values do extend up to 60 m from the contact at the FWM. Initial ε Nd and ε Hf (1.27 Ga) values within the marginal zone range from 0 to -12 and +2.1 to -15, respectively, and T CHUR model ages range from 1.13 Ga in peridotite to 2.5 Ga in contaminated gabbronorite. The adjacent country rocks have much higher ε Hf and ε Nd values at 1.27 Ga ranging from -20 to -29 and -14 to -16. The initial ε Nd values of peridotite within the marginal zone overlap those from the overlying 1500-mthick layered series [1] confirming an undepleted mantle source. The combined Hf-Nd isotopic and trace element results indicate that significant crustal contamination is restricted to a thin boundary zone especially in the WPL region. As a result, the conditions for formation of significant sulphide mineralization in the marginal zone of the Muskox intrusion are not optimal. The volume of contaminated magma was insufficient to generate a large quantity of sulphide liquid and the metal content of sulphide was limited by the inability of this sulphide liquid to interact with subsequent magma pulses. The Daduhe gold district is located in the eastern part of the Indo-Eurasian collision belt eastern margin of the Tibetan Plateau. The region is characterized by large faults and strikeslip systems related to escape tectonics during the IndoEurasia collision. A lot of gold deposits and occurrences were developed along both Jinshajiang-Ailaoshan strike-slip fault and the western margin of the Yangtze Craton, the latter is mainly includes the Daduhe, Shimian and Jinpingshan gold districts. Reference The Daduhe gold district comprises several shear-zonecontrolled Tertiary lode gold deposits. The deposits are hosted in a Precambrian granite-greenstone terrane within the Yangtze Craton. The gold mineralization occurs mainly as auriferous quartz veins with minor sulfide minerals. 18 O values (with estimated temperature ranging from 220 to 250ºC) between 7.9 and 14.7 ‰ and δD values between -39 and -108 ‰, indicating mixing of magmatic and meteoric fluids. The noble gas isotopic data, along with the stable isotopic data suggest that the ore-forming fluids have a dominantly crustal source with a significant mantle component. The volatiles in the ore-forming fluids are associated with the degassing of mantle along deep faults during the covergent tectonic environment