摘要
山西临县紫金山碱性杂岩体位于华北克拉通内鄂尔多斯盆地东缘,为汾河富碱侵入岩带中最典型的碱性岩体。岩体主要岩石类型为二长岩、辉石正长岩、霓霞钛辉岩、暗霞正长岩、霞石正长岩、假白榴石斑岩和粗面斑岩。依据SHRIMP锆石U-Pb定年结果,结合前人的测年数据,确定岩体形成于138.7~125.0 Ma,系早白垩世碱性岩浆事件的产物。该套岩石高钾、富碱、贫钙和镁,属于碱性岩石系列。稀土元素总量高,轻、重稀土元素分馏明显,Eu异常不显著。微量元素富集LREE和Rb、Ba、K、Sr等大离子亲石元素,亏损HREE和Th、U、Nb、Ta、Ti等高场强元素。同位素分析结果显示全岩ISr为0.704 228~0.706 313,εNd(t)变化于-12.3^-0.5之间。锆石的εHf(t)为-20.2^-16.4,二阶段模式年龄tDM2为2.22~2.46 Ga。推测紫金山碱性杂岩体形成于华北克拉通中生代构造体制转折后的伸展背景下,因构造体制转换产生的强大应力作用导致吕梁地区软流圈多次上涌,随后软流圈、岩石圈地幔和下地壳物质部分熔融产生的岩浆发生多次不同比例的混合,所形成的各期混合岩浆分批上侵定位在地壳浅部环境,从而形成紫金山碱性杂岩体。汾河富碱侵入岩带形成时间集中于138.7~125.0 Ma,是华北克拉通中西部岩石圈破坏事件的岩浆活动证据,其与华北克拉通东部地区几乎同时发生岩石圈减薄作用。因构造体制转换产生强大应力作用导致构造薄弱地区软流圈上涌,进而引起大规模的壳-幔岩浆相互作用,可能为华北克拉通中西部地区中生代破坏的主要原因,并且由北至南壳幔岩浆相互作用中幔源岩浆逐渐减少。
Located on the east margin of the Ordos Basin in the North China Craton, the Zijinshan alkaline com- plex, as a typical rock body in the Fenhe alkali-rich intrusive rock zone, consists of monzonite, diopside syenite, melteigite, malignite, nepheline syenite, pseudoleucite porphyry and trachyte porphyry. The results ofSHRIMP zircon U-Pb dating, combined with previous researches, suggest that the Zijinshan complex was formed between 138.7 Ma and 125.0 Ma, thus belonging to the early Cretaceous epoch. The Zijinshan complex shows high K, enrichment of alkali, and low Ca and Mg content. These rocks all show high REE content, LREE enrichment, HREE depletion and no obvious Eu anomaly. In the PM-normalized diagram, trace element pattern exhibits enrichment of LILE （Rb, Ba, K, Sr） and LREE and relative depletion of HFSE （Th, U, Nb, Ta, Ti）. Whole rocks Sr-Nd isotopic compositions are characterized by the data Isr = 0. 704 228～0. 706 313 and εNd（ t ） = - 12.3 ～ - 0.5. The monzonite shows a large variation in zircon Hf istopic compositions with eHf（t）being -20.2～ - 16.4 and two-stage model ages from 2.22 Ga to 2.46 Ca. It can be inferred that the Zijinshan complex was formed in an extension setting after Mesozoic tectonic regime inversion in the North Chi- na Craton. Continuous ashenospheric upwelling caused by the strong stress during tectonic regime inversion re- sulted in several times of mixing of melts from asthenosphere mantle, lithosphere mantle and lower crust in dif- ferent proportions, and then mixed magma of each episode migrated upward to the shallow area of the crust to form the Zijinshan complex. The Fenhe alkali-rich intrusive rock zone was formed between 138.7 Ma and 125.0 Ma. It served as the magma evidence of the destruction in the middle and west parts of the North China Craton, which happened almost at the same time as the destruction in the east part. The ashenospheric upwelling in tec- tonic weak areas caused by the strong stress during tectonic regime inversion led to the large-
出处
《岩石矿物学杂志》
CAS
CSCD
北大核心
2014年第6期1052-1072,共21页
Acta Petrologica et Mineralogica
基金
国家自然科学基金资助项目(40773041
41073047)
