Titanite-scale insights into multi-stage magma mixing in Early Cretaceous of NW Jiaodong terrane, North China Craton
Titanite-scale insights into multi-stage magma mixing in Early Cretaceous of NW Jiaodong terrane, North China Craton
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DOI：
10.1016/j.lithos.2016.04.028
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发表时间：
2016-08
期刊：
影响因子：
3.5
通讯作者：
Peng Jiang;Kui-Feng Yang;H. Fan;Xuan Liu;Ya-Chun Cai;Yue-heng Yang
中科院分区：
文献类型：
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作者：
Peng Jiang;Kui-Feng Yang;H. Fan;Xuan Liu;Ya-Chun Cai;Yue-heng Yang
胶东西北部早白垩世郭家岭式花岗闪长岩记录了华北板块破坏过程中强烈的幔壳相互作用，但其具体的成岩机制和岩浆作用过程至今仍是一个谜。火成岩中的钛铁矿可作为一种有效的成岩标志。本文对郭家岭式花岗闪长岩及其闪长质包体中的钛铁矿进行了年代学和地球化学的综合研究，以探讨其成因。花岗闪长岩（G型）和富含斜长石的闪长质包体（E型-I）中的钛铁矿具有相同的U-Pb年龄（~ 130 Ma）和难以区分的宽范围的Zr和总REE含量以及Th/U比值。然而，这两种类型的钛铁矿表现出不同的微观结构和地球化学成分。G型钛铁矿的特点是振荡环带与两个轻BSE区（LBZ）和两个或三个黑暗的BSE区，而E-I型钛铁矿的核-幔-环环带的标志。在G型钛铁矿的LBZ中，LREEs、Zr、Hf和Fe含量急剧增加，Nb、Ta、Al和F含量显著降低;而在E-I型钛铁矿中，LREEs、Zr和Hf含量从核部到地幔显著降低，F含量显著升高。基于锆在钛酸盐测温，G型钛酸盐被解释为经历了两次显着的温度升高，而E型-I钛酸盐被推断为经历了一个快速冷却过程。此外，我们认为，激烈的化学变化，在G型和E型-I的钛是由于早期岩浆混合之间的较冷的长英质岩浆和富Fe-，稀土元素的较热的闪长质岩浆。与G型和E-I型钛铁矿相比，贫斜长石闪长质包体（E-II型）中的钛铁矿以产于间隙空间为特征，具有较年轻的U-Pb年龄（~ 128 Ma），Zr、稀土总量和Th/U比值的变化范围较窄，但F含量较高（0.35- 0.76wt.%）以及极高的Nb/Ta比（高达65.6）。这种钛铁矿被认为是记录后期混合，在此期间，富F和稀土贫混合花岗闪长质岩浆挤压到不完全固结的闪长质包体伴随流体-岩石相互作用。结合前人的同位素研究结果，提出了郭家岭型花岗闪长岩的新成因模式，即在早白垩世软流圈上涌过程中，幔源镁铁质岩浆底侵作用触发了太古宙下地壳长英质岩浆与镁铁质下地壳闪长质岩浆的多阶段岩浆混合作用。这一过程进一步暗示了胶东下地壳在华北克拉通破坏过程中的再活化。
The Early Cretaceous Guojialing-type granodiorites in northwestern Jiaodong terrane carry significant records for strong mantle–crust interaction during the destruction of North China Craton (NCC); however, the definite petrogenetic mechanism and detailed magmatic process remain an enigma. Titanite in igneous rocks can serve as an effective petrogenetic indicator. Here, we present integrated geochronological and geochemical studies on titanites from Guojialing-type granodiorites and their dioritic enclaves to constrain their petrogenesis. Titanites from granodiorites (G-type) and plagioclase-rich dioritic enclaves (E-type-I) present an identical U–Pb age (~ 130 Ma) and an indistinguishable wide range of Zr and total REEs contents, and Th/U ratios. However, these two types of titanites exhibit distinct micro-scale textures and geochemical compositions. G-type titanites are characterized by oscillatory zonings with two Light BSE zones (LBZ) and two or three dark BSE zones, whereas E-type-I titanites are marked by core–mantle–rim zonings. Drastic increase of LREEs, Zr, Hf, and Fe and decrease of Nb, Ta, Al, and F contents are observed in LBZ of G-type titanites, whereas remarkable reduction of LREEs, Zr, and Hf and elevation of F contents are observed from the cores to the mantles of E-type-I titanites. Based on Zr-in-titanite thermometry, G-type titanites are interpreted to have experienced twice notable temperature increase, while E-type-I titanites are inferred to have undergone a rapid cooling process. Furthermore, we suggest that the drastic chemical changes in G-type and E-type-I titanites are ascribed to early-stage magma mixing between a colder felsic magma and a Fe-, REE-rich hotter dioritic magma. Compared to G-type and E-type-I titanites, titanites from plagioclase-poor dioritic enclaves (E-type-II) are characterized by their occurrence in interstitial space and present a relatively younger U–Pb age (~ 128 Ma) and much narrower and lower range of Zr, total REEs contents, and Th/U ratios, but reveal high F contents (0.35–0.76 wt.%) and extreme high Nb/Ta ratios (up to 65.6). Such titanites are perceived to record late-stage mingling, during which F-rich and REE-poor hybrid granodioritic magma squeezed into the incompletely consolidated dioritic enclaves with accompanying fluid–rock interaction. Combining our results with previous isotopic studies, a new genetic model for Guojialing-type granodiorites is envisaged, which involves multi-stage magma mixing between Archean lower crust-derived felsic magma and mafic lower crust-derived dioritic magma, triggered by mantle-derived mafic magma underplating during the course of asthenospheric upwelling in Early Cretaceous. Such process further implicates the reactivation of Jiaodong lower crust during the destruction of NCC.