赣-杭带早白垩世A型花岗岩成因:浅部地壳岩浆储库活化的产物

Genesis of Early Cretaceous A-type granites in Gan-Hang Belt:Generated by rejuvenile of shallow crustal magma reservoirs

A型花岗岩是一类特殊的花岗岩,其独特的矿物学和地球化学特征使其有别于与俯冲作用相关的钙碱性花岗岩。虽然前人的研究已经提出了多种A型花岗岩成因模型,但是它的成因仍然存在很大争议。本文总结了赣-杭带早白垩世火山-侵入杂岩、A型花岗岩及相关花岗岩类岩石的数据(年代学、地球化学、同位素),试图通过建立火山岩与侵入岩之间的成因联系,从而限定这些A型花岗岩的成因。对于硅质火山-侵入杂岩(例如相山火山-侵入杂岩),我们的研究表明硅质火山岩较硅质侵入岩具有更高的分异程度。硅质火山岩代表了从岩浆储库中抽离的高硅熔体,硅质侵入岩代表了岩浆储库中的残余堆晶。对于赣-杭带早白垩世A型花岗岩及相关岩石,本文根据岩石结构,将其划分为斑状-似斑状花岗岩和等粒花岗岩。岩相学及地球化学的数据表明,两类花岗岩均由来源于古老地壳物质与地幔来源基性岩浆混合所形成。而且二类花岗岩具有相似的形成时代、相似的矿物组成及类似的Sr-Nd-Hf同位素组成和连续的主微量元素变化趋势,这些特征表明它们的原始岩浆来自于同一岩浆储库,而它们岩石特征的差别是由岩浆储库内的晶体-熔体分异所主导。斑状-似斑状花岗岩以低SiO 2含量和Rb/Sr比值,高Sr、Ba含量和Eu/Eu*值为特征,代表残余的岩浆储库;等粒花岗岩具有A型花岗岩特征,并且具有极高的SiO 2含量和Rb/Sr比值,极低的Sr、Ba含量以及Eu/Eu*比值,表明它们是从岩浆储库中抽离至浅部地壳的高硅熔体。斑状-似斑状花岗岩中广泛发育暗色包体,而且等粒花岗岩中广泛发育晶洞构造,这些特征表明,幔源基性岩浆的注入和粒间熔体中挥发份的出溶作用导致先前存在的岩浆储库活化,使得高硅熔体抽离岩浆储库并向上迁移,到达地壳浅部后形成了具有A型花岗岩特征的等粒花岗岩,而残余堆晶在岩浆储库底部结晶形成了斑状-似斑状花岗岩。我们的研究表明,A型花岗岩可以通过浅部岩浆储库内晶体-熔体分离形成。

A-type granites are peculiar rocks with mineralogical and geochemical characteristics that distinguish them from subduction-related calc-alkaline granites.Although many models have been proposed by previous studies,their genesis remains highly controversial.In this study,we synthesize the data(chronology,geochemistry,and isotopes)of Early Cretaceous volcanic-intrusive complexes,A-type granites,and related granitic rocks from the Gan-Hang belt and attempt to establish the genesis connection between the volcanic rocks and intrusive rocks,and thus to constrain the origin of these A-type granites.For the volcanic-intrusive complexes(e.g.,Xiangshan volcanic-intrusive complex),our study indicates that the felsic volcanic rocks are more felsic than the felsic intrusive rocks.The felsic volcanic rocks represent the high silicic melts extracted from a magma reservoir and the felsic intrusive rocks represent residual crystal accumulation in the magma reservoir.Furthermore,A-type granite and related granitic rocks in the Gan-Hang Belt can be divided into porphyritic granite and equigranular granite.In addition,mineralogical and geochemical features of the porphyritic granite and equigranular granite indicate that they were generated by the mixing of crustal-derived felsic melts and mantle-derived mafic magmas.The porphyritic granite and equigranular granite have similar formation ages within analytic error,identical mineral assemblages,similar Sr-Nd-Hf isotopic compositions,and consistent variations in major and trace elemental compositions,which suggests that their parental magma should come from a common silicic magma reservoir and that the lithological differences are the result of melt extraction processes.Porphyritic granites are characterized by the low SiO 2 and Rb/Sr,and high Sr and Ba and Eu/Eu*,suggesting they may represent residual crystal accumulation in the magma reservoir.In contrast,the equigranular granites,which show the features of A-type granite,are characterized by the high SiO 2 and Rb/Sr,and extremely low Sr and Ba and Eu/Eu*,indicating they represent high silicic melts extracted from a magma reservoir.The wide occurrence of microgranular mafic enclaves within the porphyritic granites and miarolitic cavities in the equigranular granites reveals that the injection of mantle-derived hotter mafic magma into the magma reservoir and the exsolution of volatiles from the interstitial melt rejuvenated the pre-existing magma reservoir.Subsequent extraction and upward migration of silicic melt resulting from compaction of the magma reservoir formed the high silicic A-type granites at shallow crustal levels,which left the complementary crystal residue solidified as porphyritic granite at the bottom.Our study indicates that A-type granite can be generated in the shallow magma reservoirs via crystal-melt segregation.