ABSTRACT

Zircon U-Pb-Hf isotopes, integrated with bulk-rock major, trace element, and isotopic compositions of amphibolites from the Makran ophiolitic mélange complex (OMC), carry implications on the evolution of the Neo-Tethys Ocean. Field and petrographical-geochemical data confirm these amphibolites as products of the metamorphism of a mafic protolith. Zircons from the amphibolites have U-Pb ages of Late Early Permian (273 Ma) to Late Triassic (210 Ma), and their εHf(t) values vary from +6.32 to +15.4. The Makran amphibolites are geochemically similar to transitional- or enriched-MORBs. The 143Nd/144Nd(t) ratios for amphibolites range from 0.51260 to 0.512551, and 87Sr/86Sr(t) from 0.704433 to 0.706244. The Pb-isotope composition of these rocks is radiogenic, ranging from 15.61-15.64 for 207Pb/204Pb(t), 18.98-19.21 for 206Pb/204Pb(t) and 38.88-39.36 for 208Pb/204Pb(t). The isotopic and geochemical signatures of these rocks suggest that they were most likely formed during continental rifting and originated from a relatively enriched mantle source. This is also attested by their high zircon Ce/Ce* ratios. We interpret these amphibolites related to the Permian-Triassic plumes that triggered Gondwana rifting to open the Neo-Tethyan Ocean before metamorphosed, fragmented, and accreted into the Makran accretionary complex probably during Early to Late Cretaceous time.

摘要

锆石 U-Pb-Hf 同位素与来自 Makran 蛇绿混杂岩 (OMC) 的角闪岩的主要岩石、微量元素和同位素组成相结合，对新特提斯洋的演化具有重要意义。野外和岩石地球化学数据证实这些角闪岩是镁铁质原岩变质作用的产物。角闪岩的锆石 U-Pb 年龄为早二叠世晚期（273 Ma）至三叠世晚期（210 Ma），其 εHf(t) 值在 +6.32 至 +15.4 之间变化。Makran 角闪岩在地球化学上与过渡型或富集型 MORB 相似。角闪岩的 143Nd/144Nd(t) 比率范围为 0.51260 至 0.512551，87Sr/86Sr(t) 范围为 0.704433 至 0.706244。这些岩石的 Pb 同位素组成是放射性的，207Pb/204Pb(t) 为 15.61-15.64，206Pb/204Pb(t) 为 18.98-19.21 和 38.88-39。36 对于 208Pb/204Pb(t)。这些岩石的同位素和地球化学特征表明它们很可能是在大陆裂谷过程中形成的，并且起源于相对富集的地幔源。它们的高锆石 Ce/Ce* 比率也证明了这一点。我们解释了这些与二叠纪-三叠纪羽流相关的角闪岩，这些羽流触发了冈瓦纳裂谷，打开了新特提斯洋，然后可能在白垩纪早期到晚白垩纪时期变质、破碎并增生到 Makran 增生复合体中。