摘要
一般认为青藏高原拉萨地块后碰撞钾质-超钾质岩浆活动由西向东逐渐喷发,然而本文在拉萨地块中部麻江地区识别出一套钾质火山岩,利用单矿物金云母的40Ar-39Ar方法确定其形成于21.3Ma。这套火山岩具有高镁(>3%)和高钾(K2O/Na2O>2)等的超钾质火山岩成分特征,但其高的MgO含量是因岩石中含有后期蚀变矿物白云石所致,因此它们是具有"超钾质岩"成分特征的钾质岩。这套火山岩与拉萨地块西部地区沿着南北向地堑产出的钾质岩具有相似的构造环境,同时具有相似的地球化学特征,如富集LREE及LILE、亏损HFSE,高度富集Rb、Ba、Th、U等强不相容元素,并具有较高的(87Sr/86Sr)i比值和较低的εNd(t),指示其可能源于与古俯冲相联系的富集地幔源区。麻江地区约21Ma钾质岩的识别可能使人们重新认识有关拉萨地块后碰撞钾质-超钾质岩浆从西向东的演化特征,揭示拉萨地块后碰撞岩浆活动可能是近同时发生。同时本文认为麻江钾质岩的形成以及拉萨地块南北向裂谷系统的形成演化可能与中新世早期北向俯冲的印度岩石圈断离有关。
It is generally accepted that the post-collisional potassic and ultrapotassic magmatism are erupted from western to eastern in temporal in Lhasa block of Tibetan Plateau.However,this study has identified the potassic volcanic rocks from the Maqiang area in central of Lhasa block,which formed at ~21.3Ma by 40Ar-39Ar dating.These potassic rocks are geochemical similar to ultrapotassic rocks,e.g.,high in MgO （3%） and K2O/Na2O （K2O/Na2O2）.The high MgO contents of potassic rocks in studying area caused for late-altered dolomite and they are more similar to potassic volcanic rocks although they show some ＂ultrapotassic＂ characteristics.Geochemically and tectonically,these potassic rocks in studying area are similar to the potassic-ultrapotassic rocks occurring along N-S rifts in the western Lhasa block.The enrichment in light rare earth elements （LREE）,large ion lithophile elements （LILE,e.g.,Rb,Ba and Th） and depletion in HFSE （e.g.,Nb,Ta and Ti）,and high initial 87Sr/86Sr and low εNd（t） values of the potassic rocks in studying area,indicated that they were probably generated by partial melting of a metasomatic enrichment mantle related to early subduction events.Recognition of the potassic rocks in the Maqiang area imply that post-collisional potassic-ultrapotassic magmtism did not migrate from west to east in the Lhasa block.In other words,the post-collisional potassic and ultrapotassic lava in the Lhasa block were might erupted at the same time.And we suggest that the formation of the Miocene potassic-ultrapotassic volcanic rocks and N-S rifts in the Lhasa block were probably triggered by slab break-off of the India continent lithosphere during northern intra-continent subduction in early Miocene.
出处
《岩石学报》
SCIE
EI
CAS
CSCD
北大核心
2011年第6期1662-1674,共13页
Acta Petrologica Sinica
基金
国家重点基础研究发展规划(2009CB421004
2009CB421003)
国家自然科学基金(41073033
40930316
40872055)
中科院知识创新项目(KZCX2-YW-Q04-2)联合资助
关键词
钾质-超钾质岩
后碰撞岩浆作用
时间分布
麻江
拉萨地块
Potassic-ultrapotassic volcanic rocks
Post-collisional magmatism
Temporal distribution
Maqiang area
Lhasa block
