Petrogenesis and Geological Implications of the Oligocene Chongmuda-Mingze Adakite-Like Intrusions and Their Mafic Enclaves, Southern Tibet
Petrogenesis and Geological Implications of the Oligocene Chongmuda-Mingze Adakite-Like Intrusions and Their Mafic Enclaves, Southern Tibet
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DOI：
10.1086/667812
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发表时间：
2012-11
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影响因子：
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通讯作者：
Yuan-chuan Zheng;Z. Hou;Wei Li;W. Liang;Ke-Xian Huang;Qiuyun Li;Qing-Zhong Sun;Qiang Fu;Song Zhang
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作者：
Yuan-chuan Zheng;Z. Hou;Wei Li;W. Liang;Ke-Xian Huang;Qiuyun Li;Qing-Zhong Sun;Qiang Fu;Song Zhang
渐新世至中新世是青藏高原发育的关键时期。这种增长通常被认为是由深部地球动力学过程控制的。这一时期发育的超钾质和埃达克岩类火成岩对藏南的这些深部过程提供了制约。对藏南渐新世冲木达-明泽侵入杂岩中的镁铁质包体和容矿埃达克岩类花岗岩进行了全岩地球化学、锆石U-Pb年代学和原位锆石Hf同位素测定。赋矿岩石以花岗闪长岩和二长花岗岩为主，成分以高钾钙碱性为主。它们的全岩地球化学（MgO、Ni和Cr含量低; Nd（t）值为负值[−2.5至−3.4];低的~（87）Sr/~（86）Sr（i）值[0.7061-0.7066]）和原位锆石Hf（t）值（0.6-6.1）表明它们是由幼年下地壳部分熔融形成的，这意味着藏南地区地壳在1.30Ma前已经增厚到50 km。镁铁质包体具有典型的岩浆结构、针状磷灰石、背脉构造、淬冷边缘和结晶年龄，与寄主花岗岩一致，表明它们是岩浆成因的。镁铁质包体具有高钾-钾玄质变辉岩成分，强烈富集大离子亲石元素和轻稀土元素，亏损高场强元素，（-2.6 ~-4.9），具有较高的87 Sr/86 Sr（i）值（0.7060-0.7072），较低的锆石Hf（t）值（2.3-5.5），表明它们来自相对富集的岩石圈地幔源区。我们的新数据，加上以前发表的工作，使我们建议，在渐新世到中新世的西藏南部地区的深部地球动力学过程的特点是对流去除较低的岩石圈。藏南岩石圈根部拆沉作用引起的软流圈上涌，为藏南残余岩石圈的深熔作用提供了热量，形成了类似埃达克岩的岩石和相关的镁铁质包体。
The Oligocene to the Miocene was a critical period for the growth of the Tibetan Plateau. This growth is commonly considered to have been controlled by deep geodynamic processes. The ultrapotassic and adakite-like igneous rocks that developed during this period offer constraints on these deep-seated processes in southern Tibet. Whole-rock geochemistry, U-Pb zircon geochronology, and in situ zircon Hf isotopes have been determined for the mafic enclaves and host adakite-like granitoids in the Oligocene Chongmuda-Mingze intrusive complex of southern Tibet. The host rocks, including granodiorite and monzogranite, are mainly high-K and calc-alkaline in composition. Their whole-rock geochemistry (low MgO, Ni, and Cr contents; negative ϵNd(t) values [−2.5 to −3.4]; and low 87Sr/86Sr(i) values [0.7061–0.7066]) and in situ zircon ϵHf(t) values (0.6–6.1) indicate that they were derived by partial melting of a juvenile lower crust, implying that the southern Tibetan crust was already thickened to up to 50 km before ∼30 Ma. Mafic enclaves show typical igneous textures, acicular apatites, backveining structures, quenched margins, and crystallization ages identical to those of the host granites, indicating that they are of magmatic origin. The mafic enclaves have high-K to shoshonitic metaluminous compositions and are strongly enriched in large-ion lithophile elements and light rare earth elements, are depleted in high-field-strength elements, have negative ϵNd(t) values (−2.6 to −4.9), have relatively high 87Sr/86Sr(i) values (0.7060–0.7072), and have low zircon ϵHf(t) values (2.3–5.5), indicating that they were derived from a relatively enriched lithospheric mantle source. Our new data, together with previously published work, lead us to suggest that deep geodynamic processes below the southern Tibetan region during the Oligocene to the Miocene were characterized by convective removal of the lower lithosphere. Upwelling of the asthenosphere induced by the delamination of the southern Tibetan lithospheric root could have supplied heat to induce anatexis of the residual lithosphere of southern Tibet, generating the adakite-like rocks and related mafic enclaves there.