安徽金寨响洪甸碱性侵入岩年代学、岩石地球化学及其意义
摘要
金寨响洪甸碱性侵入岩体分布在大别造山带东北缘的北淮阳地区,侵位于上元古界青白口系佛子岭群中。主要岩石类型为正长岩(似斑状正长岩)、碱性正长岩和霞石正长岩。正长岩的主要矿物为碱性长石及少量黑云母、普通辉石、绿钙闪石;碱性正长岩包含钾长石、角闪石、辉石和少量黑云母;霞石正长岩主要由碱性长石、霞石及少量绿钙闪石、普通辉石、次透辉石等矿物组成。化学成分上均具有偏铝质、富碱、高钾、贫钛的特点。富集大离子亲石元素K、Rb、U、Nd和轻稀土元素La、Ce、Pr、Nd、Sm,亏损高场强元素Nb、Ta、P、Ti,轻、重稀土元素比值及La/Nb、Ba/Nb值较高,呈弱的负铕异常。根据高精度同位素年代学测试结果,响洪甸碱性侵入岩成岩期次分为2期:早期霞石正长岩,LA-ICP-MS锆石U-Pb年龄为(135.1±0.7)Ma;晚期正长岩,SHRIMP锆石U-Pb年龄为(125.0±1)Ma和碱性正长岩,颗粒锆石U-Pb年龄为122.3 Ma。主体岩石似斑状正长岩的Hf同位素分析结果表明,其初始176Hf/177Hf比值为0.282 243~0.282 404,εHf(t)为-10.376~-16.602,结合该岩体的Sr-Nd同位素地球化学特征,推测响洪甸碱性侵入岩是富集地幔部分熔融的产物,其源区可能有部分陆壳物质的参与。其形成的构造背景为后碰撞的伸展环境。
Lying in Beihuaiyang area on the northeastern margin of the Dabie orogenic belt,the Xianghongdian alkaline intrusion in Jinzhai County of Anhui Province wasemplaced in Upper Proterozoic strata of Foziling Group of Qingbaikou System.The complex mainly includes syenite,alkaline syenite and nepheline syenite,with the syenite(mainly porphyroid syenite)being the largest among them.Their mineral compositions are successively as follows:alkali feldspar-biotite-augite-hastingsite,potassium feldspar-hornblende-pyroxene-biotite and alkali feldspar-nepheline-hastingsite-augite-diopside.Chemically they are characterized by meta-aluminum nature and enrichment of alkaline,high K and low Ti.These alkaline rocks are also enriched in LILE elements(K,Rb,U,Nd)and LREE elements(La,Ce,Pr,Nd,Sm),depletion of HFSE elements(Nb,Ta,P,Ti),with high La/Nb,Ba/Nb ratios and slightly negative Eu anomalies.According to the isotope chronological data,the rock-forming process can be divided into two phases.The formation periods of the early nepheline syenite and the late syenite-alkaline syenite are about 135.1 Ma and 122.3 Ma~125.0 Ma respectively.Hf isotopic analysisdata of the porphyroid syenite show initial176Hf/177Hf ratios(0.282 243~0.282 404)and lowεHf(t)(-10.376~-16.602).Based on their petrogeochemistry and Hf,Sr-Nd isotopic geochemistry,itis suggested that Xianghongdian alkaline intrusion was produced by partial melting of the enrichment mantle with the addition of some crustal materials in the magma source region,and was formed in an extensional tectonic setting,mainly in the post-collision period.
Lying in Beihuaiyang area on the northeastern margin of the Dabie orogenic belt,the Xianghongdian alkaline intrusion in Jinzhai County of Anhui Province wasemplaced in Upper Proterozoic strata of Foziling Group of Qingbaikou System.The complex mainly includes syenite,alkaline syenite and nepheline syenite,with the syenite(mainly porphyroid syenite)being the largest among them.Their mineral compositions are successively as follows:alkali feldspar-biotite-augite-hastingsite,potassium feldspar-hornblende-pyroxene-biotite and alkali feldspar-nepheline-hastingsite-augite-diopside.Chemically they are characterized by meta-aluminum nature and enrichment of alkaline,high K and low Ti.These alkaline rocks are also enriched in LILE elements(K,Rb,U,Nd)and LREE elements(La,Ce,Pr,Nd,Sm),depletion of HFSE elements(Nb,Ta,P,Ti),with high La/Nb,Ba/Nb ratios and slightly negative Eu anomalies.According to the isotope chronological data,the rock-forming process can be divided into two phases.The formation periods of the early nepheline syenite and the late syenite-alkaline syenite are about 135.1 Ma and 122.3 Ma~125.0 Ma respectively.Hf isotopic analysisdata of the porphyroid syenite show initial176Hf/177Hf ratios(0.282 243~0.282 404)and lowεHf(t)(-10.376~-16.602).Based on their petrogeochemistry and Hf,Sr-Nd isotopic geochemistry,itis suggested that Xianghongdian alkaline intrusion was produced by partial melting of the enrichment mantle with the addition of some crustal materials in the magma source region,and was formed in an extensional tectonic setting,mainly in the post-collision period.
引文
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邱检生,王德滋,刘洪,凌文黎.2002.大别造山带北缘后碰撞富钾火山岩:地球化学与岩石成因[J].岩石学报,18(03):319-330.
任康绪.2003.碱性岩研究进展述评[J].化工矿产地质,25(3):151-163.
宋彪,张玉海,刘敦一.2002.微量原位分析仪器SHRIMP的产生与锆石同位素地质年代学[J].质谱学报,(1):58-62.
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吴福元,李献华,郑永飞,高山.2007.Lu-Hf同位素体系及其岩石学应用[J].岩石学报,23(2):185-220.
谢芳贵,杨祝良,沈加林,陶奎元.2000.大别山北缘中生代火山侵入杂岩地球化学特征[J].火山地质与矿产,21(3):179-188.
徐平,吴福元,谢烈文,杨岳衡.2004.U-Pb同位素定年标准锆石的Hf同位素[J].科学通报,49(14):1403-1410.
阎国翰,蔡剑辉,任康绪,牟保磊,李凤棠,储著银.2008.郯庐断裂带晚中生代Nd、Sr、Pb同位素特征及源区性质探讨[J].岩石学报,24(6):1224-1235.
张正伟,周玲棣,朱炳泉,常向阳.2002.东秦岭北部富碱侵入岩的主要矿物组成[J].矿物学报,22(1):67-74.
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赵子福,郑永飞.2009.俯冲大陆岩石圈重熔:大别-苏鲁造山带中生代岩浆岩成因[J].中国科学D辑:地球科学,39(7):888-909.
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Blichert-Toft J and Albarede F.1997.The Lu-Hf isotope geochenistry of chondrites and the evolution of the mantle-crust system:Earth Planet[J].Earth and Planerary Science Letters,148:243-258.
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Griffin W L,Wang X,Jackson S E,Pearson S E,O,Reilly S Y,Xu X S and Zhou X M.2002.Zircon chemistryand magma genesis,SE China:In-situ analysis of Hf isotopes,Tonglu and Pingtan igneous complexes[J].Lithos,61:237-269.
Griffin W L,Belousova E A and Shee S R.2004.Crustal evolution in the northern Yilarn Craton:U-Pb and H-f isotope evidence from detrital zircons[J].Precambrian Research,131(3-4):231-282.
Hacker B R,Ratschbacher L,Webb L,Mcwilliams M O,Iraland T,Calvert A,Dong S W,Wenk H R and Chateigner D.2000.Exhumation of ultrahigh-pressure continental crust in east central China:Late Triassic-Early Jurassic tectonic unroofing[J].Journalof Geophysical Research,105(B6):13339-13364.
Jahn B M,Wu F Y,Lo C Hand Tsai C H.1999.Crust-mantle-interaction induced by deep subduction of the continental crust:Geochemical and Sr-Nd isotopic evidence from post-collisionalmafic-ultramafic intrusions of the northern Dabie complex[J].ChemicalGeology,120 -176.
Leake B E,Wolley A R,ArpsC E S,et al.1997.Nomenclature of amphiboles:Report of the subcommittee on amphiboles of the international mineralogical association commission on new minerals and mineral names[J].Mineralogical Magazine,61:295-321.
Ludwig K R.2003.User's Manual for Isoplot 3.00.A geochronological toolkit for Microsoft Excel[M].Special Publication No.4a.Berkeley Geochronology Center,Berkeley,CA.
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