Early Late Cretaceous (ca. 93 Ma) norites and hornblendites in the Milin area, eastern Gangdese: Lithosphere–asthenosphere interaction during slab roll-back and an insight into early Late Cretaceous (ca. 100–80 Ma) magmatic “flare-up” in southern Lhasa (Tibet)
Early Late Cretaceous (ca. 93 Ma) norites and hornblendites in the Milin area, eastern Gangdese: Lithosphere–asthenosphere interaction during slab roll-back and an insight into early Late Cretaceous (ca. 100–80 Ma) magmatic “flare-up” in southern Lhasa (Tibet)
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DOI：
10.1016/j.lithos.2013.03.007
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发表时间：
2013-07
期刊：
影响因子：
3.5
通讯作者：
Lin Ma;Qiang Wang;Zheng‐Xiang Li;D. Wyman;Zi-qi Jiang;Jin-Hui Yang;Guo-Ning Gou;H. Guo
中科院分区：
文献类型：
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作者：
Lin Ma;Qiang Wang;Zheng‐Xiang Li;D. Wyman;Zi-qi Jiang;Jin-Hui Yang;Guo-Ning Gou;H. Guo
拉萨南部地块以侏罗纪-早始新世冈底斯岩基为主，全长500多公里，是研究西藏造山作用的理想场所。最近对冈底斯中长英质侵入岩(主要是花岗岩)的研究表明，藏南地区发生了显著的地壳增长和晚白垩世早期(约100-80 Ma)的岩浆“爆发”事件。然而，驱动这一岩浆事件的机制及其与地壳生长事件的关系尚不清楚。本文报道了拉萨南部米林地区新发现的玄武岩和含深闪长角闪岩的岩石学、年代学、地球化学和锶-钕-氢同位素资料。这些镁铁质岩石的年龄为晚白垩世早期(约93 Ma)，具有相对统一的Sr-ND-Hf同位素组成((87Sr/86Sr)i=0.7042~0.7047，εNd(T)=+2.9~+3.6，εHf(T)锆石=+10.9~+17.0)，表明它们是从相似的母岩浆演化而来的。米林闪长岩和角闪石可能分别是在岩浆演化的早期和晚期同一母岩浆的矿物分馏和堆积的产物。热力学计算表明，米林辉石具有较高的结晶温度(1240-1349℃)。根据米林玄武岩中辉石微量元素组成计算的母岩浆成分显示出轻稀土元素(LREE)配分模式([La/Yb]N=2.9-3.4；[La/Gd]N=2.1-2.9)和可变的负Nb异常([Nb/La]N=0.18-0.81)，表明它们具有双重或混合的地球化学特征。我们认为它们的母岩浆可能是软流圈上涌和交代岩石圈地幔相互作用产生的。结合中生代岩浆岩的时空分布和区域古地磁资料，进一步推测拉萨南部早白垩世-晚白垩世岩浆“爆发”也是由俯冲的特提斯大洋板块回滚引起的软流圈上涌所触发的。
At more than 500km in length, the mainly Jurassic–Early Eocene Gangdese batholith is one of the most important constituents of the southern Lhasa sub-block and provides an ideal site for study of Tibetan orogenesis. Recent studies on Gangdese intermediate-felsic intrusive rocks, mainly granites, demonstrate that remarkable crustal growth as well as an early Late Cretaceous (ca. 100–80Ma) magmatic “flare-up” event occurred in southern Tibet. However, the mechanism that drove this magmatic event and its relationship to crustal growth event are not yet clear. Here, we report detailed petrological, geochronological, geochemical and Sr–Nd–Hf isotopic data for recently identified norites and hypersthene-bearing hornblendites in the Milin area, southern Lhasa sub-block. These mafic rocks are dated at early Late Cretaceous (ca. 93Ma), and are characterized by relatively uniform Sr–Nd–Hf isotopic compositions ((87Sr/86Sr)i=0.7042 to 0.7047, εNd(t)=+2.9 to +3.6 and εHf(t)zircon=+10.9 to +17.0), suggesting that they evolved from similar parental magmas. The Milin norites and hornblendites are likely to be the products of mineral fractionation and accumulation from a common parental magma during the early and late stages of the magma evolution, respectively. Thermometric calculations indicate that pyroxenes from the Milin norites have high crystallization temperatures (1240–1349°C). The parental magmatic compositions calculated from pyroxene trace element compositions in the Milin norites show slightly flat to enriched light rare earth element (LREE) patterns ([La/Yb]N=2.9–3.4; [La/Gd]N=2.1–2.9) with variable negative Nb anomalies ([Nb/La]N=0.18–0.81), indicating their dual or hybrid geochemical characteristics. We suggest that their parental magmas may have been generated by the interaction of upwelling asthenospheric and metasomatized lithospheric mantle. Taking into account the spatial and temporal distribution of the Mesozoic magmatic rocks and regional paleomagnetic data, we further suggest that the early-Late Cretaceous magmatic “flare-up” in the southern Lhasa sub-block was also triggered by the asthenospheric upwelling, which resulted from roll-back of subducted Tethyan oceanic slab.