| 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,Ⅰtype 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.Ⅰtype belongs to a part of ophiolitic mélange.Ⅱtype 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.Ⅱtype was formed in plate subduction environments.According to petrological and geochemical study,Ⅰtype has a feature of low Si,high Fe and high Mg,and pertains to the tholeiite series;REE chondrite-normalized distribute curve indicatesⅠtype is similar to N-MORB（ΣREE>ΣN-MORB,LREE and HREE have no obvious differentiation）and has the weak-medium positive Eu anomaly;trace elements showⅠtype derived from weak loss ashospheric mantle has no negative Nb/Nb*anomaly,which suggestsⅠtype has nothing to do with subduction;tectonic environments discrimination diagrams displayⅠtype was formed from MORB;the features showⅠtype belongs to T-MORB.Ⅱtype 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）,andⅡtype has obvious negative Nb-Ta and Nb/Nb*anomaly,which suggestsⅡtype 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 withⅡtype.According to chronology research,Ⅰtype was formed in late Ordovician-middle Silurian,which is tectonic emplacement time of Wunuer-Toudaoqiao ophiolitic mélange.On the basis of two samples ofⅡtype（326±1.9Ma,MSWD=1.4,n=10）and concomitant granodiorite（323.7±1.9Ma,MSWD=0.0013,n=21）,Ⅱtype 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.Ⅰtype stands for remnant ocean crust,andⅡtype indicates subduction direction. |
