Petrogenesis of post-orogenic syenites in the Sulu Orogenic Belt, East China: geochronological, geochemical and Nd-Sr isotopic evidence
Petrogenesis of post-orogenic syenites in the Sulu Orogenic Belt, East China: geochronological, geochemical and Nd-Sr isotopic evidence
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DOI：
10.1016/j.chemgeo.2004.08.053
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发表时间：
2005-01
期刊：
影响因子：
3.9
通讯作者：
Jin-Hui Yang;Sun‐Lin Chung;S. Wilde;Fu-Yuan Wu;M. Chu;C. Lo;H. Fan
中科院分区：
文献类型：
--
作者：
Jin-Hui Yang;Sun‐Lin Chung;S. Wilde;Fu-Yuan Wu;M. Chu;C. Lo;H. Fan
中国东部苏鲁东部超高压变质造山带中的甲座山碱性杂岩由钾—超古生代辉石正长岩、石英正长岩及其伴生的基性岩脉组成。石英正长岩和矿物的SHRIMP锆石206pb /238U年龄分别为215±5 Ma和214.4±0.3和214.6±0.6 Ma，基性岩脉的就位年龄分别为200.6±0.2 Ma。这些数据表明，由于华北克拉通与扬子克拉通的大陆碰撞，甲子山杂岩在240 ~ 220 Ma的超高压变质事件后不久就位。K2O≈4.4 ~ 6.4 wt.%, K2O/Na2O≈3.5，具有较高的MgO （8.06 ~ 12.44 wt.%）、Ni （119 ~ 319 ppm）和Cr (477 ~ 873 ppm)，较低的CaO/Al2O3（~ 0.76）和TiO2（~ 1.12 wt.%）。它们还具有高Sr （87Sr/86Sr ~ 0.7073）、低Nd （Nd= ~−16.5）同位素比、富集LILE （Ba/La=66 ~ 74）、LREE [(La/Yb)N=28 ~ 33]和贫HFSE （La/Nb=4 ~ 6）。基性岩脉来源于一个难熔、再富集的岩石圈地幔源。正长岩的Sr和Nd同位素组成与基性岩脉相似，表明它们具有共同的成因。地球化学和同位素模拟表明，辉石正长岩可能是由斜辉石和橄榄石的早期分馏以及少量的地壳污染形成的，这些岩浆与基性岩脉具有相似的成分。长石为主的组合在少量污染或没有污染的情况下进行分选，形成石英正长岩。这种后造山期岩浆活动很可能形成了一个伸展环境，为华北克拉通东缘由收敛到伸展的主要地球动力学转变提供了时间约束。甲座山钾质岩浆作用和由收敛向伸展的地球动力学转变可以用下岩石圈地幔的对流移除来解释。
The Jiazishan alkaline complex in the eastern Sulu ultrahigh pressure (UHP) metamorphic orogenic belt of eastern China is composed of potassic to ultrapotassic pyroxene syenite, quartz syenite and associated mafic dikes. A SHRIMP zircon206Pb/238U age of 215±5 Ma was obtained for the quartz syenite and mineral40Ar/39Ar dating gave emplacement ages of 214.4±0.3 and 214.6±0.6 Ma for the pyroxene syenite and 200.6±0.2 Ma for the mafic dike. These dates establish that the Jiazishan Complex was emplaced shortly after the UHP metamorphic event at 240 to 220 Ma due to the continental collision between the North China and Yangtze cratons. The ultrapotassic mafic dikes, with K2O≈4.4–6.4 wt.% and K2O/Na2O≈3.5, have high MgO (8.06–12.44 wt.%), Ni (119–319 ppm) and Cr (477–873 ppm) and moderately low CaO/Al2O3(∼0.76) and TiO2(∼1.12 wt.%). They also have high Sr (87Sr/86Sr∼0.7073), low Nd (ɛNd=∼−16.5) isotopic ratios, enriched LILE (Ba/La=66–74), LREE [(La/Yb)N=28–33] and depleted HFSE (La/Nb=4–6). It appears that the mafic dikes were derived from a refractory, re-enriched lithospheric mantle source. The syenites have Sr and Nd isotopic compositions similar to the mafic dikes, implying a common origin. Geochemical and isotopic modeling suggests that the pyroxene syenites may have been generated by early fractionation of clinopyroxene and olivine, coupled with minor amounts of crustal contamination, of a mafic magma that had a similar composition to the mafic dikes. Subsequent fractionation of feldspar-dominated assemblages, with minor or no contamination, would result in the quartz syenites. This post-orogenic magmatism, resulting most likely in an extensional setting, provides time constraints on the major geodynamic transition from convergence to extension at the eastern margin of the North China craton. The Jiazishan potassic magmatism and geodynamic transition from convergence to extension can be explained by convective removal of the lower lithospheric mantle.