粤南长蛇山分异Ⅰ型花岗岩的年代学、地球化学特征及其构造意义
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摘要
在年代学、岩石学、地球化学和Sr-Nd-Hf同位素组成等研究的基础上,探讨了广东南部长蛇山花岗岩的岩石成因及其对区域晚中生代构造背景的指示意义。LA-ICP-MS锆石U-Pb测年结果显示,长蛇山花岗岩形成于中侏罗世晚期(163 Ma)。花岗岩具有富硅、富碱更富钾,贫磷,准铝-弱过铝质(A/CNK=0.98～1.11),富集大离子亲石元素(如Rb、Th、U、Pb),亏损高场强元素(如Nb、Zr、Ti)及Ba、Sr等特征。P_2O_5与SiO_2表现为负相关关系,Y与Rb表现正相关关系,总体表现出高钾钙碱性分异Ⅰ型花岗岩特征,与同期次"南岭系列"花岗岩相似。长蛇山花岗岩具有相对低的(~(87)Sr/~(86)Sr)_i值(0.705 54～0.709 40)和高的ε_(Nd)(t)值(-4.6～-0.4),相应的Nd同位素两阶段模式年龄为0.99～1.33 Ga,锆石原位Hf同位素组成变化大(ε_(Hf)(t)=-7.9～+4.5,t_(DM2)=0.99～1.81 Ga)。元素及同位素结果表明,长蛇山花岗岩可能源自中元古代古老地壳物质的部分熔融,形成过程中遭受了显著的幔源物质混染,并经历了长石、磷灰石和富钛矿物相等的分异结晶。长蛇山分异Ⅰ型花岗岩可能与"南岭系列"花岗岩具有相同的构造背景,形成于古太平洋板块俯冲作用下的弧后伸展环境。
LA-ICP-MS U-Pb zircon dating for the Changsheshan granite is(163.4±3.0) Ma and(163.2±0.6) Ma respectively, indicating that the granite was generated in the late Middle Jurassic. The granites are enriched in silicon, alkali and potassium and depleted in phosphorus, and exhibit metaluminous to weakly peraluminous, with A/CNK ratios of 0.98-1.11. They are also enriched in large ion lithophile elements, such as Rb, Th, U and Pb, and depleted in high field strength elements, such as Nb, Zr, Ti, Ba and Sr. They have relatively low 10~4×Ga/Al ratios and(Zr+Nb+Ce+Y) contents. In addition, P_2O_5 contents decrease with increasing SiO_2 contents, and Y contents decrease with increasing Rb contents. All these characteristics suggest that the Changsheshan granites are high-K calc-alkaline and highly fractionated Ⅰ-type granites, similar to the "Nanling-series" granites. The Changsheshan granites have relatively low initial(~(87)Sr/~(86)Sr) ratios(0.705 54-0.709 40) and high εNd(t) values(-4.6 to-0.4), together with highly variable ε_(Hf)(t) values(-7.9 to +4.5) and t_(DM2) model age(0.99-1.81 Ga) for the zircons. Combining with geochemical and isotopic compositions, we suggest that the Changsheshan granitic magmas were likely generated by partial melting of the Mesoproterozoic metamorphic basement, involved by small mantle-derived magmas, and that the subsequent extensive fractional crystallization of minerals(feldspar, apatite and Ti-bearing phase) produced the granites. The Changsheshan granites emplaced in the backarc extensional environment are related to subduction of the Paleo-Pacific plate, with the same tectonic settings as those of the "Nanling-series" granites.
LA-ICP-MS U-Pb zircon dating for the Changsheshan granite is(163.4±3.0) Ma and(163.2±0.6) Ma respectively, indicating that the granite was generated in the late Middle Jurassic. The granites are enriched in silicon, alkali and potassium and depleted in phosphorus, and exhibit metaluminous to weakly peraluminous, with A/CNK ratios of 0.98-1.11. They are also enriched in large ion lithophile elements, such as Rb, Th, U and Pb, and depleted in high field strength elements, such as Nb, Zr, Ti, Ba and Sr. They have relatively low 10~4×Ga/Al ratios and(Zr+Nb+Ce+Y) contents. In addition, P_2O_5 contents decrease with increasing SiO_2 contents, and Y contents decrease with increasing Rb contents. All these characteristics suggest that the Changsheshan granites are high-K calc-alkaline and highly fractionated Ⅰ-type granites, similar to the "Nanling-series" granites. The Changsheshan granites have relatively low initial(~(87)Sr/~(86)Sr) ratios(0.705 54-0.709 40) and high εNd(t) values(-4.6 to-0.4), together with highly variable ε_(Hf)(t) values(-7.9 to +4.5) and t_(DM2) model age(0.99-1.81 Ga) for the zircons. Combining with geochemical and isotopic compositions, we suggest that the Changsheshan granitic magmas were likely generated by partial melting of the Mesoproterozoic metamorphic basement, involved by small mantle-derived magmas, and that the subsequent extensive fractional crystallization of minerals(feldspar, apatite and Ti-bearing phase) produced the granites. The Changsheshan granites emplaced in the backarc extensional environment are related to subduction of the Paleo-Pacific plate, with the same tectonic settings as those of the "Nanling-series" granites.
引文
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[7] 毛建仁,厉子龙,叶海敏.华南中生代构造-岩浆活动研究:现状与前景[J].中国科学:地球科学,2014,44(12):2593-2617.
[8] 李献华,李武显,李正祥.再论南岭燕山早期花岗岩的成因类型与构造意义[J].科学通报,2007,52(9):981-991.
[9] 陈小明,王汝成,刘昌实,等.广东从化佛冈(主体)黑云母花岗岩定年和成因[J].高校地质学报,2002,8(3):293-307.
[10] 林小明,李宏卫,黄建桦,等.广东连平大顶铁矿区石背岩体 LA-ICP-MS锆石U-Pb年龄及地质意义[J].中山大学学报:自然科学版,2016,55(1):131-136.
[11] 包志伟,赵振华.佛冈铝质A型花岗岩的地球化学及其形成环境初探[J].地球与环境,2003,31(1):52-61.
[12] 庄文明,陈绍前,黄友义.佛冈复式岩体地质地球化学特征及其成岩源岩[J].广东地质,2000,15(3):1-12.
[13] 陈璟元,杨进辉.佛冈高分异I型花岗岩的成因:来自Nb-Ta-Zr-Hf等元素的制约[J].岩石学报,2015,31(3):846-854.
[14] 袁洪林,吴福元,高山,等.东北地区新生代侵入体的锆石激光探针U-Pb年龄测定与稀土元素成分分析[J].科学通报,2003,48(14):1511-1520.
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[17] Qu X,Hou Z,Li Y.Melt components derived from a subducted slab in late orogenic ore-bearing porphyries in the Gangdese copper belt,southern Tibetan Plateau[J].Lithos,2004,74(3):131-148.
[18] Chu N C.Hf isotope ratio analysis using multi-collector inductively coupled plasma mass spectrometry:An evaluation of isobaric interference corrections[J].Journal of Analytical Atomic Spectrometry,2002,17(12):1567-1574.
[19] Wu F Y,Yang Y H,Xie L W,et al.Hf isotopic compositions of the standard zircons and baddeleyites used in U-Pb geochronology[J].Chemical Geology,2006,234(1):105-126.
[20] 吴福元,李献华,郑永飞,等.Lu-Hf同位素体系及其岩石学应用[J].岩石学报,2007,23(2):185-220.
[21] Middlemost E A K.Naming materials in the magma/igneous rock system[J].Annual Review of Earth & Planetary Sciences,1994,37(3/4):215-224.
[22] Maniar P D,Piccoli P M.Tectonic discrimination of granitoids[J].Geological Society of America Bulletin,1989,101(5):635-643.
[23] Peccerillo A,Taylor S R.Geochemistry of eocene calc-alkaline volcanic rocks from the Kastamonu area,Northern Turkey[J].Contributions to Mineralogy & Petrology,1976,58(1):63-81.
[24] Sun S S,Mcdonough W F.Chemical and isotopic systematics of oceanic basalts:Implications for mantle composition and processes[J].Geological Society London Special Publications,1989,42(1):313-345.
[25] 黄会清,李献华,李武显,等.南岭大东山花岗岩的形成时代与成因:SHRIMP锆石U-Pb年龄、元素和Sr-Nd-Hf同位素地球化学[J].高校地质学报,2008,14(3):317-333.
[26] 李献华,李武显,王选策,等.幔源岩浆在南岭燕山早期花岗岩形成中的作用:锆石原位Hf-O同位素制约[J].中国科学:地球科学,2009,39(7):872-887.
[27] 孙涛,周新民,陈培荣,等.南岭东段中生代强过铝花岗岩成因及其大地构造意义[J].中国科学:地球科学,2003,33(12):1209-1218.
[28] 吴福元,刘小驰,纪伟强,等.高分异花岗岩的识别与研究[J].中国科学:地球科学,2017,47(7) :745-765.
[29] Chappell B W.Aluminium saturation in I- and S-type granites and the characterization of fractionated haplogranites[J].Lithos,1999,46(3):535-551.
[30] Whalen J B,Currie K L,Chappell B W.A-type granites:Geochemical characteristics,discrimination and petrogenesis[J].Contributions to Mineralogy & Petrology,1987,95(4):407-419.
[31] Chappell B W,Stephens W E.Origin of infracrustal (I-type) granite magmas[J].Transactions of the Royal Society of Edinburgh Earth Sciences,1988,79(2/3):71-86.
[32] Zhang Y,Yang J H,Sun J F,et al.Petrogenesis of Jurassic fractionated I-type granites in Southeast China:Constraints from whole-rock geochemical and zircon U-Pb and Hf-O isotopes[J].Journal of Asian Earth Sciences,2015,111:268-283.
[33] 刘勇,肖庆辉,耿树方,等.骑田岭花岗岩体的岩浆混合成因:寄主岩及其暗色闪长质微细粒包体的锆石U-Pb年龄和Hf同位素证据[J].地质科技情报,2011,30(2):1081-1091.
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