摘要
祁连南缘柴达木山花岗岩岩体,位于青海省大柴旦镇,岩性主要为似斑状二长花岗岩、花岗斑岩、环斑花岗岩等。本文选择代表性岩石进行了主量元素、微量元素和LA-ICP-MS锆石U-Pb年代学研究,旨在说明其岩浆成因、构造环境及意义。结果表明,柴达木山岩体K2O/Na2O比值变化为1.11~4.41,里特曼指数介于1.31~2.20,A/CNK介于1.06~1.66,地球化学特征显示,柴达木山岩体具有高钾钙碱性、过铝质、高分异I型花岗岩特征。在原始地幔标准化的微量元素分布图中,表现出不同程度的富集大离子亲石元素(LILE)Cs、Pb、K等和轻稀土元素(LREE),相对亏损高场强元素(HFSE)Y、Yb,具有明显的Ba、Nb、Ta、Sr、P、Ti负异常。稀土元素总量为207.94×10-6~418.40×10-6,LREE/HREE稀土元素之比为5.67~10.29,(La/Yb)N比值为5.88~13.84,δEu介于0.09~0.50显示负异常,δCe介于0.99~1.36,Sr/Y介于0.87~3.47,Rb/Sr介于1.22~15.45,Nb/Ta介于0.63~11.88,稀土元素球粒陨石标准化分配型式表现为轻稀土相对富集,重稀土相对亏损的右倾型且Eu负异常较明显。选取其中四个样品进行LA-ICP-MS锆石U-Pb定年,结果表明花岗斑岩(D3755-1)206Pb/238U年龄范围为400Ma^407Ma,其加权平均年龄为404.6±2.9Ma(MSWD=0.094);花岗斑岩(PM11-1)206Pb/238U年龄范围为413~424Ma,其加权平均年龄为418.4±3.0Ma(MSWD=0.30);二长花岗岩(D1028-1)的206Pb/238U年龄范围为426Ma^436Ma,其加权平均年龄为434.3±2.0Ma(MSWD=0.21);花岗闪长岩(D1506-1)的206Pb/238U年龄范围为434Ma^439Ma,其加权平均年龄为437.2±2.6Ma(MSWD=0.076),暗示柴达木山岩体的结晶年龄至少为404.6Ma^437.2Ma,形成的地质时期为早志留世-早泥盆世。祁连南缘-柴北缘早古生代岩浆活动持续时间长(372Ma^473Ma),具有多期次,主要年龄段为450~470Ma、430~450Ma、410~430Ma、400~410Ma、370~400Ma,其中430~450Ma、400~410Ma分别代表了花岗岩类侵入的两个主峰期。早期(430~470Ma)反映了南祁连洋板块向北俯冲于祁连陆块之下,柴达木陆块的继续俯冲,祁连陆块由北向南逆冲到柴达木陆块之上形成了陆陆碰撞带等一系列持续的岩浆侵入活动;晚期(370~430Ma)反映了柴达木陆块与中南祁连板块碰撞后深俯冲板块拆沉-折返及碰撞后造山带上不同块体之间的伸展、滑塌等一系列岩浆侵入活动。在空间分布上,柴北缘赛什腾山-嗷崂山代表了岩浆早期活动,主要为I型花岗岩;绿梁山-大柴旦地区-锡铁山-都兰一带,代表了岩浆晚期活动,具有I和S型花岗岩特征。柴达木山岩体代表了祁连南缘岩浆多期活动,与柴北缘岩浆活动具有同时性。
The Chaidamushan granite rock mass,in the Dachaidan Town,Qinghai Province,is mainly composed of porphyritic monzonitic granite,granite porphyry and rapakivi granite. Representative rocks were selected for main and trace element study and LA-ICP-MS zircon U-Pb geochronology to explain the magma genesis,tectonic setting and their significance. The results show that in the Chaidamushan granite the K2O/Na2O ratio changed from 1.11 to 4.41,the Litman index ranged between 1.31 to 2.20,A/CNK between 1.06 and 1.66,indicating a high potassium calc-alkaline, peraluminous and highly fractionated I-type granite. In the primitive mantle-normalized trace element distribution pattern,they show different degrees of enrichment of large ion lithophile elements(LILE) Cs,Pb,K,etc. and light rare earth elements(LREE),relative loss of high field strength elements(HFSE) Y, Yb,has obvious negative anomalies of Ba, Nb, Ta, Sr, P and Ti. The total amount of rare earth elements is 207.94×10-6~418.40×10-6,the ratio of LREE/HREE is 5.67~10.29,the ratio of(La/Yb)N is 5.88~13.84,δEu is 0.09~0.50,δCe is 0.99~1.36,Sr/Y is 0.87~3.47,Rb/Sr is 1.22~15.45,K/Rb is 0.01~0.02,and Nb/Ta is 0.63~11.88. Rare earth element chondrite-normalized distribution pattern is characterized by the relative enrichment of light rare earths,the right-dip type of heavy rare earth relative loss,and negative Eu anomaly. Four samples were selected for LA-ICP-MS zircon U-Pb dating;the results showed that the granite porphyry in D3755-1 206Pb/238U age ranged from 400 Ma to 407 Ma,and the weighted average age was 404.6±2.9 Ma(MSWD=0.094);in PM11-1 206Pb/238U age ranged from 413 Ma to 424 Ma,with a weighted mean age of 418.4±3.0 Ma(MSWD=0.30);in monzonitic granite D1028-1 206Pb/238U age ranged from 426 Ma to 436 Ma,with a weighted mean age of 434.3±2.0 Ma(MSWD=0.21);in granodiorite D1506-1 206Pb/238U age ranged from 434 Ma to 439 Ma,with a weighted mean age of 437.2±2.6 Ma(MSWD=0.076),suggesting that the crystallization age of the Chaidamushan granite is between 437.2 Ma to 404.6 Ma i.e. from the Early Silurian to the Early Devonian. The magmatism in the southern margin of the Qilianshan-North Chaidam lasted for a long time(372~473 Ma),with multiple stages dated to 450~470 Ma,430~450 Ma,410~430 Ma,400~410 Ma,370~400 Ma,amongst which 430~450 Ma and 400~410 Ma represent the two main peak periods of granite intrusion closely related to the high-pressure or ultra-high pressure metamorphism. The early period(430~470 Ma) is a response to the northward subduction of the south-Qilian ocean plate beneath the Qilian block;as the Qaidam block continued to subduct,the Qilian block from the north to the south thrusted over the Qaidam block to form a series of continuous magmatic intrusion activities in continent-continent collision zones. The late period(370~430 Ma) is a response to the deep subduction plate after the Qaidam block and the Qilian plate collided, were dismantled, and re-entered a series of magmatic intrusion activities in the stretching and sliding between different blocks. Spatial distribution of the Saishitengshan and the Aolaoshan in the northern margin of Chaidam represents the early magmatic activity,mainly I-type granite. The Luliangshan-Dachaidan-Xitieshan-Dulan area represents the late magmatic activity and has the characteristics of I-and S-type granites. The Chaidamushan rock mass represents the multi-stage magmatic activity in the southern margin of the Qilian,and is simultaneous with the magmatic activity of the northern margin of the Qaidam Basin.
作者
贺小元
杨兴科
王永
郭瑞华
廖友运
范玉海
HE Xiaoyuan;YANG Xingke;WANG Yong;GUO Ruihua;LIAO Youyun;FAN Yuhai(School of Earth Science mid Resources,Chang'an University,Xi'an,710054;Aerial Photogrammetry and Remote Sensing Bureau of China National Administration of Coal Geology,Xi'an,710054;Key Laboratory for the Study of Focused Magmatism and Giant Ore Deposits,MLR,Xi'an,710054)
出处
《地质学报》
EI
CAS
CSCD
北大核心
2020年第4期1248-1263,共16页
Acta Geologica Sinica
基金
中国地质调查局地质调查项目(编号1212011221148)和(编号12120114018213)资助成果。
关键词
柴达木山花岗岩
锆石U-PB定年
祁连南缘
I型
多期次
Chaidamushan granite
LA-ICP-MS zircon U-Pb dating
south margin of Qilianshan
I-type
multi-stage