Magma mixing origin for the post-collisional adakitic monzogranite of the Triassic Yangba pluton, Northwestern margin of the South China block: geochemistry, Sr–Nd isotopic, zircon U–Pb dating and Hf isotopic evidences
Magma mixing origin for the post-collisional adakitic monzogranite of the Triassic Yangba pluton, Northwestern margin of the South China block: geochemistry, Sr–Nd isotopic, zircon U–Pb dating and Hf isotopic evidences
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DOI：
10.1007/s00410-009-0433-2
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发表时间：
2010-03
影响因子：
3.5
通讯作者：
Jiangfeng Qin;Shaocong Lai;Chunrong Diwu;Yinjuan Ju;Yongfei Li
中科院分区：
文献类型：
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作者：
Jiangfeng Qin;Shaocong Lai;Chunrong Diwu;Yinjuan Ju;Yongfei Li
陆内高镁埃达克质岩石的成因至今仍有争议.本文报道了华南陆块西北缘阳坝一套埃达克质二长花岗岩及其基性微粒包体的主、微量元素、全岩Sr-Nd同位素、锆石U-Pb和Hf同位素数据。这些地球化学资料表明，来自加厚下陆壳的长英质埃达克质岩浆与来自陆下岩石圈地幔的镁铁质岩浆混合作用可能是陆内环境下高Mg埃达克质岩石的成因。洋坝岩体中二长花岗岩和MMEs的锆石U-Pb年龄分别为207 ± 2和208 ± 2 Ma。MMEs显示出火成岩结构，并含有丰富的针状磷灰石，表明淬火过程。它们的微量元素和演化的Sr-Nd同位素组成[（87 Sr/86 Sr）i= 0.707069-0.707138，εNd（t）= −6.5]表明起源于SCLM。来自MME的一些锆石颗粒具有正的εHf（t）值2.3-8.2，单阶段Hf模型年龄为531-764 Ma。因此，MMEs可能来自于新元古代超大陆Rodinia裂解形成的裂谷岩浆作用过程中的SCLM的部分熔融，并经历了随后的分离结晶和岩浆混合过程。寄主二长花岗岩具有典型的埃达克岩地球化学特征，高La/Yb和Sr/Y比值，低Y（9.5- 14.5ppm）和Yb含量，无明显Eu异常（Eu/Eu* = 0.81-0.90），表明部分熔融时石榴石源区稳定。其演化的Sr-Nd同位素组成[（87 Sr/86 Sr）i= 0.7041-0.7061，εNd（t）= −3.1-−4.3]和高含量的K2 O（3.22-3.84%）和Th（13.7-19.0 ppm）明显表明其来源于陆壳。此外，其高Mg#（51-55）、Cr和Ni含量可能是与SCLM源镁铁质岩浆混合的结果。埃达克质二长花岗岩中的锆石颗粒大多显示负εHf（t）值，为−9.4 ~ −0.1，两阶段Hf模式年龄为1，043 ~ 1，517 Ma;部分锆石颗粒显示正εHf（t）值，为0.1-3.9，单阶段Hf年龄为704-856 Ma。这表明埃达克质二长花岗岩源区含有新元古代幼年地壳，其在三叠纪具有正εHf（t）值。因此，陆内环境下的高镁埃达克质花岗岩是由壳源埃达克质岩浆和SCLM源镁铁质岩浆混合而成。镁铁质岩浆和埃达克质岩浆形成于晚三叠世，时间上与华南陆块北方边缘深俯冲陆壳折返一致。这种双峰式岩浆活动的发生时间晚于地幔深处的板片断裂，因此被认为是华南和华北陆块碰撞后岩石圈伸展的地球动力学响应。
Petrogenesis of high Mg# adakitic rocks in intracontinental settings is still a matter of debate. This paper reports major and trace element, whole-rock Sr–Nd isotope, zircon U–Pb and Hf isotope data for a suite of adakitic monzogranite and its mafic microgranular enclaves (MMEs) at Yangba in the northwestern margin of the South China Block. These geochemical data suggest that magma mixing between felsic adakitic magma derived from thickened lower continental crust and mafic magma derived from subcontinental lithospheric mantle (SCLM) may account for the origin of high Mg# adakitic rocks in the intracontinental setting. The host monzogranite and MMEs from the Yangba pluton have zircon U–Pb ages of 207 ± 2 and 208 ± 2 Ma, respectively. The MMEs show igneous textures and contain abundant acicular apatite that suggests quenching process. Their trace element and evolved Sr–Nd isotopic compositions [(87Sr/86Sr)i= 0.707069–0.707138, and εNd(t) = −6.5] indicate an origin from SCLM. Some zircon grains from the MMEs have positive εHf(t) values of 2.3–8.2 with single-stage Hf model ages of 531–764 Ma. Thus, the MMEs would be derived from partial melts of the Neoproterozoic SCLM that formed during rift magmatism in response to breakup of supercontinent Rodinia, and experience subsequent fractional crystallization and magma mixing process. The host monzogranite exhibits typical geochemical characteristics of adakite, i.e., high La/Yb and Sr/Y ratios, low contents of Y (9.5–14.5 ppm) and Yb, no significant Eu anomalies (Eu/Eu* = 0.81–0.90), suggesting that garnet was stable in their source during partial melting. Its evolved Sr–Nd isotopic compositions [(87Sr/86Sr)i= 0.7041–0.7061, and εNd(t) = −3.1 to −4.3] and high contents of K2O (3.22–3.84%) and Th (13.7–19.0 ppm) clearly indicate an origin from the continental crust. In addition, its high Mg# (51–55), Cr and Ni contents may result from mixing with the SCLM-derived mafic magma. Most of the zircon grains from the adakitic monzogranite show negative εHf(t) values of −9.4 to −0.1 with two-stage Hf model ages of 1,043–1,517 Ma; some zircon grains display positive εHf(t) of 0.1–3.9 with single-stage Hf ages of 704–856 Ma. These indicate that the source region of adakitic monzogranite contains the Neoproterozoic juvenile crust that has the positive εHf(t) values in the Triassic. Thus, the high-Mg adakitic granites in the intracontinental setting would form by mixing between the crustal-derived adakitic magma and the SCLM-derived mafic magma. The mafic and adakitic magmas were generated coevally at Late Triassic, temporally consistent with the exhumation of deeply subducted continental crust in the northern margin of the South China Block. This bimodal magmatism postdates slab breakoff at mantle depths and therefore is suggested as a geodynamic response to lithospheric extension subsequent to the continental collision between the South China and North China Blocks.