Petrogenetic Evolution of the Bayan Gol Ophiolite— Geological Record of an Early Carboniferous "Red Sea Type"Ocean Basin in the Tianshan Mountains, Northwestern China

The Bayan Gol ophiolite fragment is a portion of the North Tianshan Early Carboniferous ophiolite belt. This ophiolite belt represents a geological record of an Early Carboniferous 'Red Sea type' ocean basin that was developed on the northern margin of the Tianshan Carboniferous-Permian rift system in northwestern China. The late Early Carboniferous Bayan Gol ophiolite suite was emplaced in an Early Carboniferous rift volcano- sedimentary succession of shallow-marine to continental facies (Volcanics Unit). Ophiolitic rocks in the Bayan Gol area comprise ultramafic rocks, gabbros with associated plagiogranite veins, diorite, diabase, pillow basalts and massive lavas. The Early Carboniferous rifting and the opening process of the North Tianshan ocean basin produced mafic magmas in composition of tholeiite and minor amounts of evolved magmas. Compositions of trace elements and Nd, Sr and Pb isotopes reveal the presence of two distinct mantle sources: (1) the Early Carboniferous rift mafic lavas from the Volcanics Unit were generated by a relatively low degree of partial melting of an asthenospheric OIB-type intraplate source; (2) younger (late Early Carboniferous, ~324.8 Ma ago) mafic lavas from the Ophiolite Unit were formed in a relatively depleted MORB-like mantle source, located in the uppermost asthenosphere and then gradually mixed with melts from the asthenospheric OIB-like mantle. A slight interaction between asthenosphere-derived magmas and lithospheric mantle took place during ascent to the surface. Subsequently, the most depleted mafic lavas of the ophiolite assemblage were contaminated by upper-crustal components (seawater or carbonate crust).

Platinum-group Mineral(PGM) and Base-metal Sulphide(BMS) Inclusions in Chromitites of the Zedang Ophiolite, Southern Tibet, China and their Petrogenetic Significance

Voluminous platinum-group mineral （PGM） inclusions including erlichmanite （Os,Ru）S2, laurite （Ru,Os）S2, and irarsite （Ir, Os,Ru,Rh）AsS, as well as native osmium Os（Ir） and inclusions of base metal sulphides （BMS）, including millerite （NiS）, heazlewoodite （NiaS2）, covellite （CuS） and digenite （Cu3S2）, accompanied by native iron, have been identified in chromitites of the Zedang ophiolite, Tibet. The PGMs occur as both inclusions in magnesiochromite grains and as small interstitial granules between them; most are less than 10 ~m in size and vary in shape from euhedral to anhedral. They occur either as single or composite （biphase or polyphase） grains composed solely of PGM, or PGM associated with silicate grains. Os-, Ir-, and Ru-rich PGMs are the common species and Pt-, Pd-, and Rh-rich varieties have not been identified. Sulfur fugacity and temperature appear to be the main factors that controlled the PGE mineralogy during crystallization of the host chromitite in the upper mantle. If the activity of chalcogenides （such as S, and As） is low, PGE clusters will remain suspended in the silicate melt until they can coalesce to form alloys. Under appropriate conditions of fS2 and fO2, PGE alloys might react with the melt to form sulfides-sulfarsenides. Thus, we suggest that the Os, Ir and Ru metallic clusters and alloys in the Zedang chromitites crystallized first under high temperature and low fS2, followed by crystallization of sulphides of the laurite-erlichmanite, solid-solution series as the magma cooled and fS2 increased. The abundance of primary BMS in the chromitites suggests that fS2 reached relatively high values during the final stages of magnesiochromite crystallization. The diversity of the PGE minerals, in combination with differences in the petrological characteristics of the magnesiochromites, suggest different degrees of partial melting, perhaps at different depths in the mantle. The estimated parental magma composition suggests formation in a suprasubduction zone environment, perhaps in a forearc.

Oceanic Island Basalts within the West Junggar Ophiolitic Mélanges: Petrogenetic and Tectonic Implications

Recently, we focused on the Darbut and Karamay ophiolitic m41anges in West Junggar of the Central Asian Orogenic Belt （CAOB, SengOr et al., 1993; Windley et al., 2007; Xiao and Santosh, 2014）, and made much progress. This study was supported by the National Nature Science Foundation of China （No. 41303027） and Special Fund for Basic Scientific Research of Central Colleges Project （No. 2014G1271058）. The achievements are illustrated as follows.

Geochemical Characteristics of Rare Earth Elements in Guposhan Granite Complex and Their Petrogenetic Implication

In this paper the geochemical characteristics of rare earth elements in Guposhan granite complex in Nanling Area,South China,is systematically studied,and the balance of REE distribution in rock-forming minerals, which contain only 21.47～29.71 wt% of total REE in granites,is calculated.The petrogenetic relationship be- tween three different stages of the granites in the complex is discussed,and a geochemical model is suggested for the formation of granitic magma in early stage by partial melting of the crustal basement rocks containing more REE than that in the late stage.It shows that a ion-adsorption type of REE deposit could ocuur in the weath- ering crust on the early stage granite.Rb-Sr isotope isochron dating of the first stage medium-coarse-grained K-feldspar granite shows that its age is 146.5 Ma and initial ^(87)Sr/^(86)Sr ratio is 0.71198.

Petrogenetic and Metallogenetic Characteristics of the Xikeng Granitic Pegmatites, Fujian Province

The Xikeng pegmatite field lies on the eastern margin of the south China fold system in Fujian Province,and it is located at the junction of three major tectonic units. The distribution of pegmatites is obviously controlled by the fold system.There exists apparent in jectionrelationship between the pegmatites and the surrounding Sinian schist and granulitite.The granitoidsextensively distributed in the field belong either to the Variscan or to the Yenshanian cycle,and it is evidentthat the pegmatites are genetically related to Variscan migmatitic granites. The pegmatites can be grouped into four types:muscovite-orthoclase-albite pegmatite（I）.muscovitedlbite-orthoclase pegmatite（Ⅱ）,muscovite-orthoclase-albite pegmatite（Ⅲ）,and muscovite-albitespodumene pegmatite（IV）.Owing to strong metasomatism and multi-stage emplacement of pegmatitic meltsolution,the sequence of interior assemblage zones in the pegmatites does not always represent the sequenceof original crystallization. The mineral composition of the pegmatites is extremely complicated.81 kinds of minerals have so farbeen found、From type I to type IV,the mineral assemblage tend to get increasingly complex.together withthe synchronous intensification of rare-metal and Sn mineralizations.Most of the type-IV pegmatites are ofeconomic value The features of fluid inclusions in the minerals are significantly different not only in different typos ofpegmatite,but also in different parts of a single pegmatite vein.Theδ18O values of migmatitic granite andpegmatites are comparatively low（9.3-10.4‰）,and those of rock-forming fluids are higher than 9.5‰ Isotopic ages of the pegmatites brangs from 235 to 328 Ma with initial 87Sr/86Sr ratios being ”.715-0.746. According to the temporal and spatial relationships between the pegmatites and the migmatitic granitecombined with the features of the pegmatites themselves,it can be concluded that the Xikeng pegmatites arethe product of differentiation closely related to the migmatitic granite.

Petrogenetic Characterization of the Geological Formations of the Localities of Goumere-Iguela in the South West of the Bui Belt (North-East of Cote d’Ivoire)

Located in the north-east of Côte d’Ivoire, the Gouméré-Iguéla sectors were the subject of a geological mapping. These geological formations are located southwest of the Bui trench. In order to improve the petrogenetic knowledge of the study area, a multidisciplinary methodology integrating microscopic observations and geochemical analyses of major and trace elements was carried out on 15 samples considered as representative of the studied outcrops. The macroscopic and microscopic petrographic study allowed to highlight three major lithological units: 1) a volcano-plutonic unit, formed by gabbros and amphibole-pyroxenite, basalt and rhyodacite;2) a volcano-sedimentary unit containing pyroclastites (of basaltic and andesitic composition);3) a sedimentary unit (meta-sedimentary). These rocks are affected by a general metamorphism of green schist facies and a hydrothermal alteration with pervasive and vein alteration marked by the presence of quartz, calcite, epidote, chlorite and sericite. From a geochemical point of view, the results obtained indicate that the plutonites are gabbros while the volcanites and volcanosedimentary rocks have compositions of basalts, basaltic andesites, rhyolites and dacites. They have mainly a tholeitic character related to subduction zones with crustal contamination. The metasediments are arenites emplaced in an active continental margin environment.

U-Pb dating of early Mesozoic granodioritic intrusions in southeastern Hunan Province, South China and its petrogenetic implications

The NE-trended Mesozoic granodioritic intrusions are spatially and temporally associated with the copper multi-metal mineralization in southeastern Hunan Province, South China. U-Pb dating result of single-grained zircons of four samples respectively from Shuikoushan, Baoshan, western Tongshanling and eastern Tongshanling intrusions reveals that their crystallization age spans a range from 172 Ma to 181 Ma, which also represents the oldest age of the regional copper multi-metal mineralization. Some of the zircon grains give an upper intercept age of about 1753 Ma and 207Pb/206Pb apparent age of (1752 ± 4) Ma, implying the involvement of the pre-Cambrian metamorphic (possible Middle Proterozoic) basement in their genesis. The presence of such a kind of zircon grains in these granodiorites indicates either that the parental magmas were assimilated by basement rocks during magma ascent or that lower/middle crustal rocks were one of the important components during the melting process.

Discussion on measurement and calculation principle of petrogenetic depth

Geologic and geophysical data certificate that lithosphere and its down rock behave as solid, therefore,it is better to measure petrogenetic and metallogenetic depth according to internal stress of solid states. Lithomechanic experiment found that the bigger the confining pressure, the bigger the tensile and shear strength of rocks,so rock can bear very high differential stress but it is not broken. Sibson put forward that the property of rocks under 10-15km of the surface changes from brittle to ductile (rather than from elastic to plastic). Rock with ductile de-

REE geochemistry of some alkali-rich intrusive rocks in China

Based on the REE analyses of about 300 samples from 40 alkali-rich intrusive rocks, a systematic study on the features of REE composition was made, and the major factors controlling the REE distribution are discussed. According to REE, trace elements composition and Sr, Nd isotopic characteristics, the source materials and petrogenetic processes of the alkali-rich intrusive rocks were studied. The petrogenetic models of Zijinshan, Saima, Nianzishan and Shanhaiguan alkali-rich intrusive rocks were calculated using multisequence fractional crystallization and assimilation fractional crystallization models.