摘要
富碱侵入岩具有深源浅成的属性,是示踪地幔组成的窗口。位于东天山与塔里木克拉通北缘交界处的阔克塔格以西出露一处以正长岩为主体、包含少量中—基性岩石单元的富碱侵入岩。LA-ICP-MS锆石U-Pb测年显示其侵位于227~224 Ma之间,为印支期岩浆作用产物。该岩体具有富碱(Na_(2)O+K_(2)O=7.93%~12.28%)、富铝(Al_(2)O_(3)=15.62%~18.67%)、贫镁(MgO=0.12%~4.01%)、贫钛(TiO_(2)=0.14%~1.63%)的特征,属于准铝质的(A/CNK=0.78~1.00)高钾钙碱性—钾玄岩系列。微量元素表现出高场强元素Nb、Ta、Th、U、Zr、Hf的富集,不相容元素Sr、P、Ti的亏损特征;轻重稀土分馏明显((La/Yb)N=15.09~34.90),表现为轻稀土强烈富集的右倾型曲线,Eu异常变化较大(δEu=0.12~1.72)。在同位素组成上,富碱侵入岩呈现相对宽泛但总体富集的全岩Sr-Nd和锆石Hf同位素特征,(^(87)Sr/^(86)Sr)i=0.70489~0.70581,ε_(Nd)(t)=14.56~6.74,ε_(Hf)(t)=11.48~+3.06,锆石δ^(18)O值(5.49‰~6.87‰)略高于地幔值(5.3‰±0.6‰)。阔克塔格西富碱侵入岩具A1型花岗岩特征,形成于板内伸展构造环境中。基于主、微量元素及Sr-Nd-Hf-O同位素特征,笔者认为该杂岩体为同源岩浆演化产物,源于富集岩石圈地幔的原始岩浆上升并经历AFC(同化混染-分离结晶)过程。晚三叠世塔里木克拉通北缘—东天山—北山地区处于板内构造体制,A1型花岗岩和正长岩差异化的时空分布特征很可能记录了古亚洲洋自西到东“剪刀式”的闭合过程。
Derived from the deep lithospheric source and emplaced at shallow crustal levels,alkali-rich intrusive rocks are regarded as a window for deciphering the nature of the mantle.An alkaline composite pluton composed of predominant syenite and subordinate intermediate-mafic rocks is exposed in the Kuoktagh area,the transition region between the eastern Tianshan Belt and the north margin of Tarim craton.New LA-ICP-MS zircon ages range from~227 to~224 Ma,indicating Indosinian emplacement.Samples from the pluton are enriched in total alkali(Na_(2)O+K_(2)O=7.93%~12.28%),Al(Al_(2)O_(3)=15.62%~18.67%),and poor in Mg(MgO=0.12%~4.01%),Ti(TiO_(2)=0.14%~1.63%).They exhibit a metaluminous affinity(A/CNK=0.78~1.00)and belong to high-K calc-alkaline to shoshonitic series.In terms of trace element patterns,the studied rocks are characterized by the enrichment in high-field-strength element(HFSE),such as Nb,Ta,Th,U,Zr,and Hf,and depletion in Sr,P,and Ti.The studied samples possess strong enrichment in light rare earth elements(LREE)relative to heavy rare earth elements(HREE),with variable Eu anomalies(δEu=0.12~1.72).Samples from the pluton show variable but overall“ancient”(enriched)whole-rock Sr-Nd and zircon Hf isotopic results((^(87)Sr/^(86)Sr)_(i)=0.70489~0.70581,ε_(Nd)(t)=14.56~6.74,ε_(Hf)(t)=11.48~3.06),and have zirconδ^(18)O values(5.49‰~6.87‰)slightly higher than that of the mantle(5.3‰±0.6‰).Considering petrological and geochemical features of the studied pluton,combined with other geological evidence,we suggest that the studied alkaline rocks are of A1-type granite and were formed in an intra-plate extensional setting.Taking into account the characteristics of major elements,trace elements and Sr-Nd-Hf-O isotopes,we propose that the composite pluton is the product of assimilation and fractional crystallization(AFC),and the primary magma was derived from an enriched lithosphere mantle.North margin of Tarim craton,eastern Tianshan Belt and Beishan region were likely together in an intra-plate,anorogenic tectonic framework.The temporospatial distribution of A-type granites in these regions might record the closure history of the Paleo-Asian Ocean from west to east via a‘scissor-like’model.
作者
潘蓓蓓
黄河
张招崇
王涛
吴欢欢
秦切
李海舟
PAN Beibei;HUANG He;ZHANG Zhaochong;WANG Tao;WU Huanhuan;QIN Qie;LI Haizhou(School of Earth Sciences and Resources,China University of Geosciences(Beijing),Beijing 100083,China;Institute of Geology Chinese Academy of Geological Sciences,Beijing 100037,China)
出处
《地质学报》
EI
CAS
CSCD
北大核心
2023年第7期2195-2221,共27页
Acta Geologica Sinica
基金
国家自然科学基金项目(编号41830216,42173052)资助的成果。
关键词
富碱侵入岩
阔克塔格西
三叠纪
地球化学
岩石成因
构造意义
alkali-rich intrusive rocks
western Kuoktagh
Indosinian period
geochemistry
petrogenesis
tectonic significance
