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Ao C.,Jilin University | Ao C.,Tianjin Institute of Geology and Mineral Resources | Sun F.,Jilin University | Li B.,Jilin University | And 4 more authors.
Geotectonica et Metallogenia | Year: 2015

The basic-ultrabasic rocks in the Eastern Kunlun area not well documented mainly because of the harsh natural environment. Researches on petrography, geochronology, geochemistry and tectonic setting of the Xiaojianshan gabbro will be of significance for the understanding of mantle magmatism in East Kunlun. LA-MC-ICP-MS zircon U-Pb dating shows that the gabbro in the Xiaojianshan area was formed at ~227 Ma. The rocks are calc-alkaline and characterized by low SiO2 (43.72%- 50.92%, average value is 47.07%), high Mg# (60-75), high V (68.0×10-6- 182×10-6) and Cr (60.0×10-6- 430×10-6) contents, and low Sr (456×10-6- 600×10-6, average value is 526×10-6), Rb/Sr ratio (0.03~0.10, average value is 0.06). The rocks have low REE contents, slightly enriched in LREE with positive Eu anomalies (ΣREE=13.6×10-6-62.5×10-6, (La/Yb)N=3.95-6.17, δEu=1.08-1.68), besides, all rocks are enriched in LILE (Rb and Ba), incompatible elements (Th and U) and depleted in HFSE (Nb and Ta). Geochemical features of the rocks indicate that the Xiaojianshan gabbro was originated from partial melting of the metasomatized lithospheric mantle caused by the dehydration of the subducting slab. Combined with regional tectonic setting, we propose that the Xiaojianshan gabbro is mantle origin formed in extensional regime of the Late Indosinian. © 2015, Science Press. All right reserved.

Wang G.,Jilin University | Sun F.,Jilin University | Li B.,Jilin University | Li S.,Qinghai Geological Survey | And 3 more authors.
Earth Science Frontiers | Year: 2014

In this paper, we present petrographical, zircon U-Pb dating and geochemical data for the mafic-ultramafic intrusion in Xiarihamu Cu-Ni deposit to constrain its formation time, petrogenesis and the tectonic setting. Xiarihamu No.1 mafic-ultramafic intrusion locates in the middle of the Eastern Kunlun middle uplifted basement and granitic belt, south of the North Kunlun Fault. The intrusion covers an area of 0.7 km2, in shape of a flat lopolith, with the strikes of NEE. The intrusion is mainly composed of gabbro-norite, orthopyroxenite, olivine pyroxenites, plagio-lherzolite and orthopyroxene peridotite. Pyroxenite and peridotite are the main host rocks for the Cu-Ni ores. Major element contents of Xiarihamu complex show sub-alkalic series and are characterized by low SiO2 contents (SiO2 36.68%-52.58%), low TiO2 contents (TiO2 0.13%-0.47%) and alkali contents (Na2O+K2O 0.26%-1.95%), but high MgO contents (MgO 10.91%-35.81%). The m/f values of the rocks range from 3.88 to 6.29, falling into the range of ferrous-ultramafic rocks. Similar LREE-rich pattern ((La/Yb)N=1.44-2.98) and no Eu anomaly indicate that Xiarihamu complex have the same magma source. Trace elements are enriched in LILE (Rb, Th, U, K) and relatively depleted in HFSE (Nb and Ta). The presence of Neoproterozoic granite relicts and the La/Yb, Ce/Yb, Th/Yb, Nb/La, and La/Sm values suggest the limited crustal contamination during the rise of the magma. Zircon LA-MC-ICP-MS U-Pb dating of gabbro-norite yielded 423±1 Ma, MSWD=0.14, i.e., the Late Silurian. Zircon 176Hf/177Hf values are in the range of 0.282628-0.282833 with corresponding εHf(t) values of 4.0-10.9. Lu-Hf single-stage modal ages (tDM1) vary from 610 to 875 Ma and mean age (788 Ma), which is older than U-Pb age. We suggest that the primary magma derived from a depleted mantle, and may have been added metasomatic enriched lithospheric mantle components, and contaminated by crustal material. Combined with regional tectonic evolution, we believe that the intrusion formed in the post-collisional extension setting, perhaps triggered by slab break-off. The fractional crystallization of olivine and orthopyroxene and addition of crustal S appear to be directly related to trigger S saturation.

Wang G.,Jilin University | Sun F.,Jilin University | Li B.,Jilin University | Li S.,Qinghai Geological Survey | And 3 more authors.
Geotectonica et Metallogenia | Year: 2013

In this paper, we discuss the zircon U-Pb age, major and trace element geochemistry of the syenogranite intrusion in the north of the Xiarihamu ore district. Zircon U-Pb dating demonstrates that the weighted mean 206Pb/238U age for the zircons from the syenogranite is 391.1±1.4 Ma (MSWD=0.06), i.e., the Early Devonian. The syenogranite is slightly peraluminous and belongs to the high-K calc-alkaline series. The syenogranite is characterized by high SiO2 (75.55%~76.10%), high alkali (Na2O+K2O=8.23%~8.46%), high FeOT/MgO (FeOT/MgO=17.40~42.59), low CaO (0.54%~0.69%) and MgO (0.03%~0.09%) contents. The syenogranite is characterized by a "sea-gull" REE pattern and significant negative Eu anomaly (δEu=0.09~0.12), as well as enrichment of Ga (24.3×10-6~26.9×10-6), Zr (132.0×10-6~363.0×10-6) and Y (86.1×10-6~97.0×10-6), and depletion of Sr (8.6×10-6~19.5×10-6) and Ba (14.0×10-6~37.9×10-6). And thus, the syenogranite belongs to aluminous A-type granite. The high Rb/Sr (ranging from 14.97 to 38.26, and the average value is 22.63) and Rb/Nb ratios (ranging from 13.84 to 16.13, and the average value is 14.54) of the rock indicate a crustal origin. Combined with the regional tectonic evolution and geochemical discrimination, we suggest that the syenogranite was formed in a post-orogenic setting. During the Late Silurian-Early Devonian, the tectonic regime in the East Kunlun area changed from compression to extension due to asthenosphere upwelling. Under the extensional setting, the upwelling asthenosphere provided enhanced heat and triggered the partial melting of the overlying felsic crust, and produced the A-type syenogranite.

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