Age and geochemistry of western Hoh-Xil-Songpan-Ganzi granitoids, northern Tibet: Implications for the Mesozoic closure of the Paleo-Tethys ocean
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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藏北可可西里松潘甘孜西部花岗岩的年龄和地球化学：对古特提斯洋中生代闭合的启示
DOI：
10.1016/j.lithos.2013.12.019
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发表时间：
2014-03
期刊：
影响因子：
3.5
通讯作者：
Alan Chapman
中科院分区：
文献类型：
--
作者：
Ren-Deng Shi;Mihai N. Ducea;Paul Kapp;Alan Chapman
为了更好地了解古特提斯洋闭合晚期岩浆的成因和演化，对西藏北方可可西里-松潘-甘孜杂岩进行了地质调查。HXSG杂岩是由大量的中-上三叠统海相重力流沉积组成的，这些沉积广泛地被火成岩侵入。这些出露的早中生代岩石记录了青藏高原陆块拼合过程中的增生构造历史。八个岩体样品从西部HXSG复杂产量锆石U-Pb年龄范围为225至193 Ma。虎洞梁岩体和卓乃湖岩体的白云母40 Ar/39 Ar年龄分别为210.7 ± 2.5Ma和212.7 ± 2.5Ma。这些深成岩可细分为两个地球化学性质不同的组。第1组（221-212 Ma：大鹏湖、长虹湖和黑石北湖岩体）由高钾钙碱性岩石组成，稀土元素分异强烈，（La/Yb）N比值高（91-18），普遍缺乏Eu异常（Eu*/Eu = 1.02-0.68）。在原始地幔标准化蛛网图上，第1组岩石显示出明显的负Nb-Ta和Ti异常。第1组岩石具有高Sr（782-240 ppm）和低Y（6.3-16.0 ppm）含量以及高Sr/Y比值（84-20）。根据Sr-Nd-Hf同位素数据（87 Sr/86 Sri = 0.7079-0.7090，εNd（t）= − 7.7-− 4.7，εHf（t）= − 5.7-− 0.8）和低MgO含量（MgO = 1.10-2.18%），第1组岩石在地球化学上与埃达克质岩石相似，可能是由下行古陆的部分熔融形成的。特提斯洋板块和上覆海洋沉积物。第2组岩体（225-193 Ma：大黑山、云雾岭、卓乃湖、马栏山和虎洞梁岩体）表现出低P2 O 5、高SiO2的特征，为中钾-高钾I型钙碱性岩体，具有低Sr（14-549 ppm）和高Y（22.3- 10.5ppm）的特征。第2组岩石具有不同的稀土元素分馏模式（（La/Yb）N= 3-38）和负Eu异常（Eu*/Eu = 0.02-0.86）。结合Sr-Nd-Hf同位素（87 Sr/86 Sri = 0.7072-0.7143，εNd（t）= − 6.6-− 2.0，εHf（t）= − 0.6-+ 3.0），第2组岩石很可能是由年轻地壳源的部分熔融形成的。总的来说，这些数据表明，可可西里浊积岩下面的大陆弧地壳比以前认为的。我们认为，俯冲的古特提斯洋板的回滚导致了在HXSG重力流沉积之下发育的上覆大陆弧碎片的部分熔融。
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. Muscovite40Ar/39Ar ages for the Hudongliang and Zhuonai Lake plutons yield ages of 210.7 ± 2.5 Ma and 212.7 ± 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)Nratios (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/86Sri= 0.7079–0.7090, εNd(t) = − 7.7–− 4.7, εHf(t) = − 5.7–− 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 P2O5with increasing SiO2and 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/86Sri= 0.7072–0.7143, εNd(t) = − 6.6–− 2.0, εHf(t) = − 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.
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