北苏鲁威海地区共生伟晶岩脉-钙硅酸盐岩的形成机制和地质意义

Formation mechanism and geological implications of co-existing pegmatite veins-calc-silicate rocks in the Weihai area, northern Sulu orogenic belt

威海位于苏鲁造山带最北端,出露片麻岩和榴辉岩等多类超高压岩石,是研究大陆俯冲带变质、深熔和熔体效应的理想靶区。基于详细野外观察,本文对该区的一套钙硅酸盐岩、伟晶岩脉和寄主片麻岩进行了岩相学、地球化学和锆石年代学等研究,旨在深入探究超高压变质岩的多期熔融及与之伴随的地质过程。阴极发光显示,三类岩石中的锆石整体具有核-边结构,继承核年龄为859±20Ma~272±6Ma,与苏鲁变质岩内大多残余岩浆锆石的年龄一致。新生锆石共记录了约237±4Ma、224±7Ma~218±2Ma和177±5Ma~166±3Ma等三组年龄。结合不同微区的稀土元素特征,本文认为,237±4Ma记录了峰期变质时代,而224±7Ma~218±2Ma和177±5Ma~166±3Ma年龄的锆石呈现典型的深熔锆石特征,分别代表俯冲陆壳折返至高压榴辉岩相和加厚地壳初始伸展阶段的部分熔融年代。伟晶岩与片麻岩中继承锆石的Hf同位素组成不同,表明形成伟晶岩的熔体并非寄主岩石直接熔融而来。此外,钙硅酸盐岩及其内部的单斜辉石具有极低的Cr、Ni等相容元素含量和负Eu异常特征,且均显示大离子亲石元素富集、高场强元素亏损的特点。并且,钙硅酸盐岩与苏鲁超高压片麻岩的Sr-Nd同位素组成接近,其内部继承/新生锆石与伟晶岩中相应锆石微区的Hf同位素组成一致。综合分析上述数据并结合前人成果,本文推测,钙硅酸盐岩为深俯冲陆壳折返阶段长英质熔体与片麻岩内部的大理岩团块发生熔岩反应所致。综上,本研究进一步约束了俯冲陆壳在折返及造山垮塌阶段两期熔融的时间及熔体来源等信息,并为大陆俯冲带深部的熔体效应和物质循环提供了参考。

Situated at the northeastern terminus of the Sulu orogenic belt, Weihai boasts a diverse array of ultrahigh-pressure (UHP) metamorphic rocks, including gneiss and eclogite, rendering it an ideal locale for investigating metamorphism, anatexis, and the ramifications of melting in continental subduction zones. Based on detailed field observation, this paper presents the results of an integrated study of petrology, geochemistry and zircon geochronology on a suite of calc-silicate rocks, pegmatite and the host gneisses in Weihai. The primary aim is to advance our comprehension of multistage partial melting processes of UHP rocks and the associated geological phenomena. Cathodoluminescence imaging reveals a core-rim structure in the analyzed rocks. The inherited zircon cores have 206Pb/238U dates ranging from 859±20Ma to 272±6Ma, consistent with the ages of relict igneous zircons found in UHP rocks within the Sulu belt. On the other hand, the newly formed zircon domains display three distinct metamorphic age groups: 237±4Ma, 224±7Ma~218±2Ma, and 177±5Ma~166±3Ma. Based on the rare earth element characteristics of these zircon domains, the age of 237±4Ma is linked to peak UHP metamorphism, while the zircon domains dated at 224±7Ma~218±2Ma and 177±5Ma~166±3Ma align with typical anatectic zircons, signifying the periods of partial melting of the subducted continental crust during high-pressure eclogite-facies exhumation and initial orogenic collapse, respectively. Distinct Hf isotope compositions in inherited zircon domains within the pegmatite compared to those in the host gneiss suggest a non-direct origin of the melts responsible for pegmatite formation from the host rocks. Moreover, both clinopyroxene and the calc-silicate rocks exhibit low contents of compatible elements such as Cr and Ni, negative Eu anomalies, and enrichment in large ion lithophile elements alongside depletion in high field strength elements. The calc-silicate rocks display enrichment of Sr-Nd isotopic compositions akin to UHP gneisses in the Sulu belt, and the Hf isotope compositions of inherited/newly formed zircon domains resemble those of the pegmatite. By synthesizing these findings with previous research, we posit that the calc-silicate rocks originated from the interaction of felsic melts derived from the continental crust during exhumation with small marble blocks enclosed within UHP gneisses. Overall, this research provides further insights into the timing and sources of hydrous melt during two-stage melting processes associated with exhumation and collapse of the subducted continental crust, shedding light on melt effects and mass cycling at depth in continental subduction zones.