Late Silurian mafic and felsic magmatism in the South Qilian Belt, northern Tibet Plateau: Response to slab breakoff
Late Silurian mafic and felsic magmatism in the South Qilian Belt, northern Tibet Plateau: Response to slab breakoff
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藏北高原南祁连带晚志留世镁铁质和长英质岩浆作用：对板片断裂的响应
DOI：
10.1016/j.lithos.2020.105860
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发表时间：
2020-10
期刊：
影响因子：
3.5
通讯作者：
Wang Chao
中科院分区：
文献类型：
--
作者：
Zhu Xiaohui;Chen DanLing;Ren Yunfei;Yang Meng;Wang Chao
柴达木北带（NQDB）是位于柴达木地块与南祁连带（SQB）之间的早古生代陆相超高压（UHP）变质带，其超高压变质岩具有差时/过时的挖掘和深溶历史。为约束NQDB及邻区多期发掘及伴生岩浆作用，利用单键dpb和单键dhf同位素对其进行了岩石学、地球化学和锆石综合研究。类oibb白云石富集轻稀土元素（lree）、大离子亲石元素（LILEs）和高场强元素（hfse），贫重稀土元素（hree），结晶年龄为425±7 Ma，锆石εHf(t)值为−4.2 ~ 0.3。考虑到本区早古生代发生了洋陆深俯冲，在俯冲带上方金红石不稳定场中，洋壳与幼年岩石圈地幔之间的壳幔相互作用可能形成了obb类玄武岩的肥沃地幔源。a型英安岩斑岩富集Al、Fe、LILEs和lree，贫Ca、Mg、Ba、Sr、Eu和HFSEs，具有较高的Zr浓度（231 ~ 271 ppm）， 1万*Ga/Al比值（2.51 ~ 2.73），结晶温度（738 ~ 836℃）等特征。利用单键dpb和单键dhf同位素数据显示，英安岩斑岩结晶温度为423±5 Ma，锆石εHf(t)值在−8.9 ~−3.3之间。继承的岩浆锆石形成年龄为472±8 Ma， εHf(t)值在- 3.9 ~ - 1.9之间，与北秦岭洋俯冲相关岩浆活动同时期，表明英安岩斑岩可能来源于早古生代弧。结合前人对NQDB的超高压变质作用（约458 ~ 420 Ma）、逆变质作用（约432 ~ 397 Ma）和深熔作用（约436 ~ 417 Ma）的研究，认为洋岩石圈与大陆岩石圈的断裂引发了约425 Ma软流圈上升流的热扰动，并导致了丰富的交代地幔和早古生代弧的部分熔融，形成了SQB的oibb型白云岩和a型英安岩斑岩。此外，同时期的镁质和长英质岩浆作用表明了一个重要的构造转变，即俯冲的大陆地壳在约425 Ma之前经历了局部剥离/拆离，而在板块断裂之后经历了整体拆离。
The North Qaidam Belt (NQDB) is an Early Paleozoic continental ultrahigh-pressure (UHP) metamorphic belt between the Qaidam Block and South Qilian Belt (SQB) in northwest China, and the UHP metamorphic rocks recorded differential/diachronous exhumation and anatexis histories. To constrain the multi-stage exhumation and the associated magmatism of the NQDB and its adjacent areas, an integrated study of petrology, geochemistry, zircon Usingle bondPb and Lusingle bondHf isotopes was conducted on the OIB-like dolerite and A-type dacite porphyry from the SQB. The OIB-like dolerite is enriched in light rare-earth elements (LREEs), large ion lithophile elements (LILEs), and high field strength elements (HFSEs), and depleted in heavy rare-earth elements (HREEs), with a crystallization age of 425 ± 7 Ma and zircon εHf(t) values ranging from −4.2 to 0.3. Given that the oceanic/continental deep subduction occurred in this area during Early Paleozoic, the fertile mantle source of OIB-like dolerite was likely formed by the crust-mantle interaction between the oceanic crust and juvenile lithospheric mantle at the instability field of rutile above the subduction zone. The A-type dacite porphyry is enriched in Al, Fe, LILEs and LREEs, depleted in Ca, Mg, Ba, Sr, Eu and HFSEs, and is characterized by high Zr concentrations (231–271 ppm), 10,000*Ga/Al ratios (2.51–2.73) and crystallization temperatures (738–836 °C). Zircon Usingle bondPb and Lusingle bondHf isotopic data reveal the dacite porphyry crystalized at 423 ± 5 Ma with zircon εHf(t) values ranging from −8.9 to −3.3. The inherited magmatic zircons gave a formation age of 472 ± 8 Ma with εHf(t) values ranging from −3.9 to −1.9, contemporaneous with the oceanic subduction-related magmatism in the NQDB, indicating the dacite porphyry may be derived from the Early Paleozoic arc. Combined with the previous studies on the UHP metamorphism (ca. 458–420 Ma), retrograde metamorphism (ca. 432–397 Ma), and anatexis (ca. 436–417 Ma) in the NQDB, we conclude that the breakoff of the oceanic lithosphere from the continental lithosphere triggered the thermal perturbation from the asthenosphere upwelling at ca. 425 Ma and the subsequent partial melting of the fertile metasomatized mantle and Early Paleozoic arc, forming the OIB-like dolerite and A-type dacite porphyry in the SQB. Moreover, the coeval mafic and felsic magmatism indicates an important tectonic transformation that the subducted continental crust experienced a localized detachment/exhumation before ca. 425 Ma and entire exhumation after slab breakoff.
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