Age and geochemistry of western Hoh-Xil-Songpan-Ganzi granitoids, northern Tibet: Implications for the Mesozoic closure of the Paleo-Tethys ocean

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• 作者：Li-Yun Zhang^a ; ^{zly@itpcas.ac.cn" class="auth_mail} ; Lin Ding^a ; Alex Pullen^b ; ^c ; Qiang Xu^a ; De-Liang Liu^a ; Fu-Long Cai^a ; Ya-Hui Yue^a ; Qing-Zhou Lai^a ; Ren-Deng Shi^a ; Mihai N. Ducea^b ; ^d ; Paul Kapp^b ; Alan Chapman^e
• 关键词：Hoh-Xil&ndash ; Songpan-Ganzi ; Tibet ; Granitoids ; Paleo-Tethys ; Rollback
• 刊名：Lithos
• 出版年：March, 2014
• 年：2014
• 卷：190-191
• 期：Complete
• 页码：328-348
• 全文大小：4702 K

文摘

A geologic investigation was undertaken in the Hoh-Xil-Songpan-Ganzi (HXSG) complex, northern Tibet in order to better understand magma genesis and evolution during the late stages of Paleo-Tethys ocean closure. The HXSG complex is composed of vast accumulations of Middle-Upper Triassic marine gravity flow deposits that were extensively intruded by igneous rocks. These early Mesozoic rocks exposed in this area record a rich history of accretionary tectonics during the amalgamation of the Tibetan Plateau terranes. Eight plutons sampled from the western HXSG complex yield zircon U-Pb ages that range from 225 to 193 Ma. Muscovite ⁴⁰Ar/³⁹Ar ages for the Hudongliang and Zhuonai Lake plutons yield ages of 210.7 卤 2.5 Ma and 212.7 卤 2.5 Ma, respectively. These plutonic rocks can be subdivided into two geochemically distinct groups. Group 1 (221-212 Ma: Dapeng Lake, Changhong Lake and Heishibei Lake plutons) is composed of high-K calc-alkaline rocks that have strongly fractionated REE patterns with high (La/Yb)_N ratios (91-18) and generally lack Eu anomalies (Eu*/Eu = 1.02-0.68). Rocks in Group 1 display pronounced negative Nb-Ta and Ti anomalies on primitive mantle-normalized spidergrams. Group 1 rocks exhibit high Sr (782-240 ppm) and low Y (6.3-16.0 ppm) contents with high Sr/Y ratios (84-20). Based on Sr-Nd-Hf isotopic data (⁸⁷Sr/⁸⁶Sr_i = 0.7079-0.7090, 蔚Nd(t) = 鈭?#xA0;7.7-鈭?#xA0;4.7, 蔚Hf(t) = 鈭?#xA0;5.7-鈭?#xA0;0.8) and low MgO contents (MgO = 1.10-2.18%), Group 1 rocks are geochemically similar to adakitic rocks and were probably derived from partial melting of the downgoing Paleo-Tethys oceanic slab and overlying marine sediments. Group 2 plutons (225-193 Ma: Daheishan, Yunwuling, Zhuonai Lake, Malanshan and Hudongliang plutons) display lower P₂O₅ with increasing SiO₂ and are medium-K to high-K I-type calc-alkaline bodies with low Sr (14-549 ppm) and high Y (22.3-10.5 ppm) contents. Group 2 rocks have variable fractionated REE patterns ((La/Yb)_N = 3-38) and negative Eu anomalies (Eu*/Eu = 0.02-0.86). Together with Sr-Nd-Hf isotopes (⁸⁷Sr/⁸⁶Sr_i = 0.7072-0.7143, 蔚Nd(t) = 鈭?#xA0;6.6-鈭?#xA0;2.0, 蔚Hf(t) = 鈭?#xA0;0.6-+ 3.0), Group 2 rocks are most likely formed by partial melting of the juvenile crustal sources. Collectively, these data suggest that the Hoh-Xil turbidites were underlain by more continental arc crust than previously thought. We propose that rollback of the subducting Paleo-Tethys oceanic slab led to partial melting of overlying continental arc fragments which developed beneath the HXSG gravity flow deposits.