川西里伍矿田两期石榴子石Sm-Nd等时线年龄及成因OA

The Liwu Ore Field,situated in the core of the Jianglang Dome on the southeastern margin of the Songpan-Ganze orogenic belt,is the second-largest copper-polymetallic resource base in Sichuan Province.These high-grade stratiform copper deposits are hosted within the Neoproterozoic metamorphic series of the Liwu Complex,where two types of garnet with distinct modes of occurrence have been identified:(1)Fine-grained,deformed garnets that are barren of mineralization.These garnets,with grain sizes commonly ranging from 1 to 5 mm,are oriented along the S2 foliation and are ubiquitous in the wall rocks of the Liwu Complex.(2)Coarse-grained,undeformed garnets that are associated with mineralization.These garnets,with grain sizes commonly ranging from 5 to 25 mm,cross-cut the S2 foliation and are exclusively found within the deepest mineralized alteration zones.To precisely determine the formation age and genesis of the garnets,this study conducted electron probe microanalysis(EPMA)for major element compositions and Sm-Nd isotopic dating.The analytical results indicate that the fine-grained and deformed garnets are primarily composed of almandine(with an average weight percentage of 82.08%),grossular(avg.8.04%),and pyrope(avg.8.30%),yielding a Sm-Nd isochron age of(185±29)Ma(MSWD=0.55,n=4).In contrast,the coarse-grained and undeformed garnets are mainly composed of almandine(avg.71.35%),grossular(avg.8.34%),pyrope(avg.6.73%),and spessartine(avg.13.55%),yielding a Sm-Nd isochron age of(168±17)Ma(MSWD=2.0,n=4).Integrated with previous research findings,we propose that the fine-grained and deformed garnets are of metamorphic origin,formed during ca.204～180 Ma peak metamorphism of the Songpan-Ganze orogenic belt.Conversely,the coarse-grained and undeformed garnets are of hydrothermal origin,directly related to ca.164～159 Ma magmatic-hydrothermal activity in the Jianglang Dome.Based on available exploration data,we suggest that the stratigraphic horizon enriched in coarse-grained and undeformed garnets likely represents the main magmatic-hydrothermal pathway for epigenetic mineralization in the Liwu Ore Field,implying that the mineralization potential of the underlying deeper strata may be limited.