岩浆储库的活化:以浙江西部马头石英二长岩为例

Rejuvenation of magma reservoir:A case study of the Matou quartz monzonite in western Zhejiang

东南沿海地区发育大量早白垩世晚期到晚白垩世早期的碱性岩,主要为石英正长岩和石英二长岩。然而,这些岩石的成因一直都存在争议。为了明确这些岩石的成因,本论文选择了浙江西部的马头岩体进行研究,分析了其中石英二长岩全岩主微量元素组成和矿物相特征,并结合锆石U-Pb年代学、锆石Hf同位素、锆石微量元素、磷灰石微量元素和斜长石电子探针等技术对其进行了详细研究。锆石U-Pb定年结果显示,马头岩体形成于100±1Ma,属于早白垩世晚期岩浆活动产物。野外观察发现,马头岩体中发育大量的闪长质暗色包体。马头石英二长岩中的锆石ε_(Hf)(t)(-10.4~-0.4)变化极大,表明其是由古老陆壳来源的酸性岩浆与地幔来源基性岩浆混合所形成。马头岩体中的锆石结构十分复杂,在CL图像上普遍显示出黑色的核部、白色的幔部以及黑色的边部。最早期的黑色锆石核部具有相对高的Th、U、Hf含量,变化较大的Eu/Eu^(*)(0.5~0)、Zr/Hf(40~70)和Th/U比值、Ti含量,表明它们结晶自地壳中的演化程度较高的低温且结晶度较高的硅质岩浆储库。白色的幔部锆石具有低的Th、U、Hf含量,Eu/Eu^比值相对较高(0.15~0.37),表明由于基性岩浆的注入导致先存岩浆储库发生活化和熔融,使得岩浆储库内温度升高且熔体比例增加,而随着矿物进一步结晶,晶体间隙之中的熔体演化程度升高,其中结晶了最晚期的具有高U和Hf含量、低Eu/Eu和Zr/Hf的黑色锆石边部。斜长石电子探针数据表明,随着斜长石斑晶的结晶,岩浆储库经历了晶体-熔体分离过程,导致活化后储库内的熔体从富钙向富钠演化,最终结晶形成了细粒状的石英和钠长石。石英二长岩中的磷灰石具有较低的Sr含量(113×10^(-6)~417×10^(-6)),较高的U、Y、轻稀土和较低的重稀土含量,表明先存岩浆储库在经历活化之后,也经历了斜长石、榍石和磷灰石等矿物的分离结晶。锆石记录了岩浆储库的活化和熔融过程,而斜长石和磷灰石等矿物仅记录了岩浆储库活化后的晶体-熔体分离和熔体抽离过程。因此,马头石英二长岩表明其代表岩浆储库活化后经历了熔体抽离的残余岩浆储库,与之同时形成的高硅流纹岩(或高硅花岗岩)是从岩浆储库中抽离出的富硅熔体。最后,我们研究表明岩浆储库在大部分时间内处于低温、低熔体比例的状态,基性岩浆的注入使得先存岩浆储库经历了活化和熔融,这是浅部地壳岩浆储库能再次经历晶体-熔体分离和火山喷发的关键因素。

Previous studies indicate that significant amount alkaline rocks,primarily consisting of quartz syenite and quartz monzonite,were formed during the late Early Cretaceous to early Late Cretaceous in the coastal area of southeastern China.However,their origin is still controversy.To determine the genesis of these rocks,this study focused on the quartz monzonite in the Matou pluton in western Zhejiang and conducted detailed investigations of whole-rock major and trace element analysis,mineralogical scanning,zircon U-Pb geochronology,zircon Hf isotopes,zircon trace elements,apatite trace elements and plagioclase electron probe analysis on it.The zircon U-Pb dating reveals that the Matou pluton formed at 100±1Ma,representing a product of magmatic activity during the late Early Cretaceous.Field observations found abundant mafic enclaves within the Matou pluton.Combining the zircon Hf isotopic data(ε_(Hf)(t)=-10.4~-0.4),it is suggested that the Matou pluton resulted from the magma mixing of felsic magmas derived from ancient continental crust and mafic magmas derived from the mantle.Zircons in the Matou monzonite show complex textures,commonly displaying a black core,a white mantle,and black rims in CL(cathodoluminescence)images.The earliest black zircon cores exhibit relatively high Th,U,Hf contents,varying Eu/Eu^*(0.50~0),Zr/Hf(40~70)and Th/U ratios,as well as Ti content.This suggests that they crystallized from a high-degree,low-temperature,and highly crystalline silicic magma reservoir in the crust.The white mantle zircons have lower Th,U,Hf contents,and relatively higher Eu/Eu ratios(0.15~0.37)which indicates that injection of mafic magma caused reactivation and melting of pre-existing magma reservoirs,leading to an increase in temperature and melt proportion within the reservoir.As minerals continued to crystallize and felsic melt extraction occurred,the remaining felsic melt within the magma reservoir underwent significant evolution.This led to the crystallization of the latest-stage black zircon rims characterized by high U and Hf contents and low Eu/Eu^(*) and Zr/Hf ratios.Electron probe data on plagioclase suggests,as plagioclase phenocrysts crystallized,the magma reservoir underwent a crystal-melt separation process,causing the evolution of melt from being rich in calcium to being rich in sodium.Ultimately,this led to the crystallization of fine-grained quartz and sodium feldspar.Apatite in the quartz monzonite has low Sr content(113×10^(-6)~417×10^(-6)),high U,Y and light rare earth element,and low heavy rare earth element contents.This implies that,after reactivation,the pre-existing magma reservoir also underwent crystal fractionation of minerals such as plagioclase,titanite and apatite.Zircon records the reactivation and melting processes of the magma reservoir,while minerals like plagioclase and apatite only record the crystal-melt separation and melt extraction processes after the reactivation of the reservoir.Therefore,the Matou quartz monzonite suggests that it represents a residual magma reservoir that underwent melt extraction after reactivation,with contemporaneous high-silica rhyolite(or high-silica granite)representing silica-rich melt extracted from the magma reservoir.Finally,our study indicates that the magma reservoir predominantly existed in a low-temperature,low-melt proportion state for most of the time.The injection of mafic magma led to the reactivation and melting of the pre-existing magma reservoir,which is a crucial factor enabling shallow crustal magma reservoirs to undergo crystal-melt separation and volcanic eruptions once again.