The Characteristics And Geological Significance,Two Types Of The Late Paleozoic Basic Intrusive Rocks In Yakeshi Region

When 1:25,0000 regional geological survey work carried out detailedly in Yakeshi region,we found two different genetic kinds of basic intrusive rocks exposed.Distributed in Wunuer-Toudaoqiao fault zone,?type is composed of gabbro,gabbro-diabase,disbase-porphyrite?disbase dyke?,most of which spread in the shape of residuum or fault block,and is associated with meta-basalt and radiolarian silicalite.?type belongs to a part of ophiolitic mélange.?type in Baijingshan and Wuerqihan chiefly consists of gabbro and gabbro-diorite,most of which spread in the shape of stock and drip stone,and is associated with quartz diorite,tonalite,granodiiorite and monzonitic granite.?type was formed in plate subduction environments.According to petrological and geochemical study,?type has a feature of low Si,high Fe and high Mg,and pertains to the tholeiite series;REE chondrite-normalized distribute curve indicates?type is similar to N-MORB??REE>?N-MORB,LREE and HREE have no obvious differentiation?and has the weak-medium positive Eu anomaly;trace elements show?type derived from weak loss ashospheric mantle has no negative Nb/Nb^*anomaly,which suggests?type has nothing to do with subduction;tectonic environments discrimination diagrams display?type was formed from MORB;the features show?type belongs to T-MORB.?type has a feature of low-medium Si,medium Fe,medium Mg,high Al and K₂O/Na₂O<0.5,and pertains to the calcium alkaline rock series;REE chondrite-normalized distribute curve leans overtly right?LREE have obvious differentiation and HREE have no obvious one?and has no Eu anomaly;primitive mantle normalized trace elements spider diagrams leans overtly right?enriched LILE,and depleted HFSE?,and?type has obvious negative Nb-Ta and Nb/Nb^*anomaly,which suggests?type formed ocean plate of partial melting with a little mantle wedge has a connection with subduction;concomitant granodiorite has a feature of high Si,low Fe,low Mg,high Al,K₂O/Na₂O<0.5,obviously high Sr,low Y and low Yb;genetic discrimination diagrams belonging to O-adakitic rocks show concomitant granodiorite has evolution trend of partial melting MORB,which is associated with?type.According to chronology research,?type was formed in late Ordovician-middle Silurian,which is tectonic emplacement time of Wunuer-Toudaoqiao ophiolitic mélange.On the basis of two samples of?type?326±1.9Ma,MSWD=1.4,n=10?and concomitant granodiorite?323.7±1.9Ma,MSWD=0.0013,n=21?,?type formed in early carboniferous said the beginning of the subduction.Recognizing two kinds of basic intrusive rocks clarifies evolution process of ocean basin and subduction,and provides the basis of establishing tectonic framework and geological evolution in Yakeshi region.?type stands for remnant ocean crust,and?type indicates subduction direction.