西藏措勤县雄玛地区始新统林子宗群帕那组火山岩地球化学特征及构造背景

Lithochemical,Geochemical,Characteristics and Tectonic Setting of the Volcanic Rocks in the Eocene Pana Formation,Linzizong Group, in the Xiongma Area,Coqen County,Xizang( Tibet)

西藏措勤县雄玛地区林子宗群帕那组火山岩大地构造位置位于冈底斯—念青唐古拉山板片,地层分区上属于冈底斯地层区、隆格尔—南木林地层分区。岩性主要为一套以浅灰色、灰绿色流纹质凝灰岩为主,夹浅黄绿色、灰绿色流纹岩及英安岩、火山角砾岩、杂砂岩及砾岩的岩石组合。之下与永珠组和拉嘎组呈喷发不整合接触。岩石地球化学特征表明该火山岩系属过铝质的高钾钙碱性—亚碱性系列,稀土元素地球化学中轻稀土元素明显富集,微量元素中大离子亲石元素(Rb、K、Th、U、LREE)富集,高场强元素(Nb、P、Ti)亏损。以上特征说明雄玛地区林子宗群帕那组火山岩岩浆源于陆壳重熔,属与俯冲造山有关的陆相火山岩,同时又兼具陆—陆碰撞的特点。推测帕那组火山岩形成于新特提斯洋壳俯冲基本结束,印度板块与欧亚板块碰撞开始的构造背景下,年龄大概在38Ma左右,为始新世晚期。

The volcanic rocks of the Pana Formation in Xiongma area, Coqen county, Xizang （Tibet） are situated at Gangdis--Nianqingtangula plate, whose classification of stratigraphic region is belongs to Longger-- Nanmulin stratigraphic subregion of Gangdis stratigraphic region. The rocks are mainly composed of medium acidic volcanic lava and volcaniclastic rocks, with a few tuffaceous sandstone, sandy conglomerate. The lower end of the Pana Formation is unconformable contact with the Yongzhu Formation or Laga Formation. Petrogeochemical characteristics show that the volcanic series belongs to Peraluminous high potassium calc alkali--alkaline series. The geochemical characteristics of REE show that the volcanic rocks belong to LREE enrichment type, enriched in LILE （Rb, K, Th, U, LREE） and depleted in HFSE （Nb, P, Ti） in trace element. The above features show that the volcanic rocks of the Pana Formation, Linzizong Group, in Xiongma area, are derived from continental crust remehing, belong to continental volcanic rocks related to subduction orogenism. At the same time, with the characteristics of the land--continental collision. Speculated that the volcanic rocks of the Pana Formation formed in the tectonic background when the neo-Tethys oceanic crust subduction is finished and the Indian plate and Eurasian plate collision, time was about 38 Ma, i.e. late Eocene.