陳志洪,邢光福,姜 楊,匡福祥

(南京地質(zhì)礦產(chǎn)研究所,江蘇 南京 210016)

華夏陸塊是中國(guó)東部大陸重要的組成部分之一,了解其早期的形成與演化對(duì)充分認(rèn)識(shí)中國(guó)大陸地殼的形成、構(gòu)造格局的演變乃至全球構(gòu)造事件均具有重要意義(舒良樹,2006;于津海等,2007;鄭永飛和張少兵,2007)。經(jīng)過(guò)長(zhǎng)期的研究積累,特別是近十年來(lái),隨著同位素測(cè)年技術(shù)(SHRIMP和LA-ICP-MS)迅猛發(fā)展,對(duì)華夏陸塊變質(zhì)基底的地層、構(gòu)造、巖石、同位素年代學(xué)及陸殼深部結(jié)構(gòu)等研究取得了很大的進(jìn)展,提供了更多可靠“古陸塊”存在的證據(jù)(Li et al.,2000,2005;Wan et al.,2007;Xu et al.,2007;Xia et al.,2012;Yu et al.,2008,2009,2012;Chen and Xing,in press)。但是,由于區(qū)域前寒武紀(jì)地質(zhì)構(gòu)造的復(fù)雜性,以及顯生宙以來(lái)歷次構(gòu)造事件的疊加改造,制約了對(duì)早期構(gòu)造巖漿熱事件性質(zhì)的深入理解。因此,尋找相關(guān)事件記錄的地質(zhì)體就是破解這一難題的關(guān)鍵。筆者最近在對(duì)出露于北武夷淡竹地區(qū)前寒武紀(jì)基底巖石進(jìn)行的專題研究中,首次識(shí)別出了古元古代 A型流紋斑巖,這為深入研究華夏陸塊基底物質(zhì)組成和早期構(gòu)造演化提供了新的資料。

1 區(qū)域地質(zhì)概況與樣品特征

研究區(qū)地處武夷地塊北部的浙西南地區(qū)(圖1)。區(qū)域上出露的前寒武紀(jì)變質(zhì)巖系大致可以劃分為上、下兩套巖石地層單元:下部為古元古代的八都群,主要巖性為黑云斜長(zhǎng)片麻巖、變粒巖、黑云片巖和斜長(zhǎng)角閃巖,經(jīng)歷了角閃巖相的中高溫區(qū)域變質(zhì)作用和較強(qiáng)烈的混合巖化作用(胡雄健等,1992;甘曉春等,1995;Yu et al.,2012);上部為新元古代的龍泉群,主要巖性為變粒巖、云母片巖、綠簾斜長(zhǎng)角閃巖、含鐵石英巖和大理巖等,經(jīng)歷了高綠片巖相到低角閃巖相的區(qū)域變質(zhì)作用(金文山等,1992;陳正宏等,2008;Li et al.,2010)。

圖1 浙西南-閩西北地質(zhì)簡(jiǎn)圖(底圖據(jù)Li et al.,2005;Wan et al.,2007)Fig.1 Geological sketch map of SW Zhejiang and NW Fujian provinces (modified from Li et al.,2005;Wan et al.,2007)

本次研究新發(fā)現(xiàn)的流紋斑巖位于浙江省仙居縣淡竹鄉(xiāng)龍竹村,與八都群變質(zhì)巖呈侵入接觸關(guān)系(圖2a);流紋斑巖主要零星(小于 0.01 km2)出露于龍竹村旁的一條河溝內(nèi),呈肉紅色,斑狀結(jié)構(gòu),具片狀-片麻狀的變形特征。斑晶總量約10%～15%,主要為熔蝕的高溫石英,普遍有較寬的反應(yīng)邊且波狀消光(圖2b,d);另有少量斜長(zhǎng)石斑晶(碳酸鹽化和絹云母化)和褐簾石化的黑云母(圖2c)。基質(zhì)具有霏細(xì)結(jié)構(gòu),未能辨析其礦物組成(圖2d)。

2 分析方法

鋯石用人工重砂方法選出,然后在雙目鏡下挑純,選出晶形較好、具代表性的鋯石粘貼在環(huán)氧樹脂表面,拋光后將待測(cè)鋯石進(jìn)行陰極發(fā)光(CL)圖像分析。CL照相在中國(guó)科學(xué)院地質(zhì)與地球物理研究所電子探針實(shí)驗(yàn)室完成。鋯石U-Pb測(cè)年和微量元素分析在合肥工業(yè)大學(xué)資源與環(huán)境工程學(xué)院完成,采用的儀器型號(hào)為 Agilent 7500a,激光剝蝕系統(tǒng)為Coherent Inc公司生產(chǎn)的ComPex102 ArF準(zhǔn)分子激光剝蝕系統(tǒng)。分析時(shí)激光束斑直徑為32 μm,激光脈沖重復(fù)頻率為6 Hz。應(yīng)用Nist610玻璃作為微量元素外標(biāo),鋯石標(biāo)樣 91500進(jìn)行同位素分餾校正,鋯石標(biāo)樣Mud Tank作為同位素監(jiān)控樣,實(shí)驗(yàn)原理和詳細(xì)的測(cè)試方法見(jiàn)閆峻等(2012)。ICP-MS的分析數(shù)據(jù)通過(guò) ICPMSDataCal程序計(jì)算獲得同位素比值、年齡和誤差(Liu et al.,2008,2010)。普通鉛校正采用Andersen (2002)的方法進(jìn)行,校正后的結(jié)果用ISOPLOT程序(ver 2.49)完成年齡計(jì)算和諧和圖的繪制(Ludwig,2001)。

鋯石Hf同位素分析在中國(guó)地質(zhì)科學(xué)院礦產(chǎn)資源研究所實(shí)驗(yàn)室Thermo Fisher Neptune多接收電感耦合等離子質(zhì)譜和New Wave UP213激光剝蝕系統(tǒng)上進(jìn)行。激光剝蝕直徑為 44 μm,激光脈沖重復(fù)頻率為 8 Hz。測(cè)定時(shí)采用鋯石國(guó)際標(biāo)樣GJ-1作為參考物質(zhì),儀器的運(yùn)行條件及詳細(xì)的分析流程見(jiàn)Wu et al.(2006)和候可軍和袁順達(dá)(2010)。分析過(guò)程中GJ-1的176Hf/177Hf加權(quán)平均值為(0.282012±12)(2SD,n=15),與推薦值(候可軍和袁順達(dá),2010)在誤差范圍內(nèi)完全一致。εHf(t)的計(jì)算利用176Lu的衰變常數(shù)為1.865×10-11/a(Scherer et al.,2001),球粒隕石現(xiàn)今值176Hf/177Hf=0.282772和176Lu/177Hf=0.0332(Blichert-Toft and Albarede,1997);虧損地幔 Hf模式年齡計(jì)算采用當(dāng)前虧損地幔值176Hf/177Hf=0.28325和176Lu/177Hf=0.0384(Griffin et al.,2000);二階段 Hf模式年齡采用上地殼平均值176Lu/177Hf=0.015(Griffin et al.,2002)。

圖2 淡竹流紋斑巖的野外地質(zhì)及鏡下特征Fig.2 Photos and microphotographs for the rhyolitic porphyries in the Danzhu region

全巖主量元素在南京地質(zhì)礦產(chǎn)研究所實(shí)驗(yàn)測(cè)試中心采用荷蘭Panalytical公司生產(chǎn)的Axios型波長(zhǎng)色散X射線熒光光譜儀測(cè)定,分析精度優(yōu)于5%。微量元素在南京地質(zhì)礦產(chǎn)研究所實(shí)驗(yàn)測(cè)試中心采用 Finnigan ELEMENT2型電感耦合等離子質(zhì)譜(ICP-MS)測(cè)定,分析精度優(yōu)于5% (Rudnick et al.,2004)。

3 結(jié) 果

3.1 鋯石LA-ICP-MS U-Pb年齡和稀土元素

對(duì)淡竹流紋斑巖(樣品ZJ-11-1)中的25顆鋯石共計(jì)完成了25個(gè)測(cè)點(diǎn)的LA-ICP-MS分析,U-Pb同位素組成列于表1。標(biāo)定的測(cè)年鋯石大部分為長(zhǎng)柱狀,長(zhǎng)/寬比為 2∶1～3∶1,顆粒大小約100～300 μm。樣品鋯石普遍受到了后期變質(zhì)作用改造而發(fā)生重結(jié)晶,發(fā)育一定的核邊結(jié)構(gòu),具核部較黑且環(huán)帶結(jié)構(gòu)不清晰(圖3a)。測(cè)年結(jié)果顯示,23顆鋯石分析點(diǎn)的Th(77×10-6～ 433×10-6)和U(108×10-6～2024×10-6)含量相對(duì)較高,相應(yīng)的Th/U比值范圍為0.25～1.62,屬于典型的巖漿鋯石(吳元保和鄭永飛,2004),給出的207Pb/206Pb的年齡范圍為1902～1776 Ma。在諧和圖上,23個(gè)分析點(diǎn)構(gòu)成一條很好的不一致線,上下交點(diǎn)的年齡分別為 1844±26 Ma和157±650 Ma(MSWD=0.91)。23個(gè)分析點(diǎn)給出的207Pb/206Pb年齡加權(quán)平均值為(1819±16 Ma)(MSWD=0.91),代表了流紋斑巖的成巖年齡(圖3b)。另外 2顆鋯石的 Th(12×10-6～15×10-6)和U(134× 10-6～186×10-6)的含量較低,Th/U比值變化于 0.08～0.09,可能代表變質(zhì)成因(吳元保和鄭永飛,2004),給出的206Pb/238U年齡分別為240±6 Ma(5號(hào)分析點(diǎn))和238±6 Ma(6號(hào)分析點(diǎn))。

23個(gè)巖漿型鋯石測(cè)點(diǎn)在進(jìn)行U-Pb同位素分析時(shí),同步原位測(cè)定了鋯石稀土元素的含量(表2)。結(jié)果顯示,該類鋯石具有較高的 REE總量(∑REE=309×10-6～1112×10-6),在鋯石稀土元素球粒隕石標(biāo)準(zhǔn)化配分圖解上(圖3c),該類鋯石極度富集 HREE,呈左傾型,并具明顯的正Ce異常(Ce/Ce*=6.8～90.6)和負(fù) Eu異常(Eu/Eu*=0.02～0.24),符合一般巖漿鋯石的稀土元素特征(Belousova et al.,2002)。另外2個(gè)變質(zhì)鋯石的測(cè)點(diǎn),由于當(dāng)時(shí)儀器運(yùn)行狀態(tài)不佳,未能獲得其理想數(shù)據(jù)。

3.2 全巖元素地球化學(xué)特征

4組流紋斑巖樣品的全巖主量、微量與稀土元素的成分見(jiàn)表3。它們與典型的A型花崗巖在地球化學(xué)組成上非常類似(Whalen et al.,1987),具高 Si(SiO2=67.0%～72.1%),富堿(Na2O+K2O=8.1%～9.0%),富 Fe(FeOT=2.9%～5.8%),顯著貧 Mg(MgO=0.19%～0.42%)和鈣(CaO=0.97%～1.7%)的特征。在 K2O-SiO2圖解中,全部樣品落入鉀玄巖系列中(圖略)。它們的Al2O3含量為13.2%～14.0%,A/CNK值為0.99～1.08,顯示出準(zhǔn)鋁質(zhì)-弱過(guò)鋁的特征。流紋斑巖的稀土元素總量較高,∑REE =626×10-6～765×10-6,輕重稀土之間分異明顯(圖4a),(La/Yb)N=14.3～28.1,Eu虧損強(qiáng)烈,Eu/Eu*為0.29～0.42。微量元素方面,流紋斑巖的 Ga含量較高(22.1×10-6～28.9×10-6),10000Ga/Al值為 3.0～4.1;HFSE元素組合(Zr+Nb+Ce+Y)的值也很高,為 880×10-6～999×10-6。在原始地幔標(biāo)準(zhǔn)化蛛網(wǎng)圖上(圖4b),所有樣品均表現(xiàn)為明顯的Rb、Th、U和Pb峰以及Ba、Sr、Nb、Ta和(Ti+Eu)谷的分布特征。

表1 淡竹流紋斑巖的鋯石U-Pb同位素分析數(shù)據(jù)Table1 LA-ICP-MS zircon U-Pb results for the rhyolitic porphyry in the Danzhu region

圖3 淡竹流紋斑巖的鋯石CL圖像(a),鋯石U-Pb年齡諧和圖(b)和稀土元素球粒隕石配分曲線圖(c)(內(nèi)圈(小)為L(zhǎng)A-ICP-MS靶位,外圈(大)為Hf同位素激光剝蝕靶位;球粒隕石值取自Sun and McDonough,1989)Fig.3 Cathodoluminescence (CL) images of the representative zircons from the Danzhu rhyolitic porphyry(a) (with small and large white circles showing areas for U–Pb and Lu–Hf isotope analysis,respectively),U-Pb concordia diagram for zircons from the Danzhu rhyolitic porphyry(b) and chondrite-normalized REE patterns for zircons from the Danzhu rhyolitic porphyry(c) (chondrite and primitive mantle values are from Sun and McDonough (1989))

3.3 鋯石Hf同位素組成

在U-Pb年齡的基礎(chǔ)上,選擇15個(gè)巖漿鋯石分析點(diǎn)進(jìn)行了 Hf同位素測(cè)定,結(jié)果列于表4。鋯石176Lu/177Hf比值介于 0.000345～0.000817之間,176Hf/177Hf比值介于 0.281405～0.281588,單階段鋯石Hf模式年齡tDM為2.31～2.55 Ga,兩階段鋯石Hf模式年齡t2DM為2.46～2.76 Ga。鋯石Hf同位素初始比值εHf(t)為-8.4～-2.2。

4 討 論

4.1 巖石類型與成因

最早Loiselle and Wones(1979)將A型花崗巖定義為堿性(alkaline)、貧水(anhydrous)和非造山(anorogenic)的花崗巖,它一般是堿過(guò)飽和而鋁不飽和。但近年來(lái)的研究表明,A型花崗巖不僅包括堿性巖類,還擴(kuò)大到鈣堿性、弱堿、準(zhǔn)鋁、弱過(guò)鋁甚至強(qiáng)過(guò)鋁質(zhì)巖石(吳福元等,2007)。同時(shí),一些流紋巖也顯示出A型花崗巖的特征,通常認(rèn)為它們是A型花崗巖噴出相的產(chǎn)物(Li et al.,2005)。淡竹流紋斑巖具有A型花崗巖的地球化學(xué)特征:(1)高Si富堿,高K貧Al、Ca和Mg,巖石的FeOT/MgO值(13.8～15.1)明顯高于I、S和M型花崗巖的平均值,與典型的A型花崗巖一致(～13.4) (Whalen et al.,1987);(2)富集大離子親石元素(Rb、Th、U)和高場(chǎng)強(qiáng)元素Zr和Hf,虧損 Ba、Sr、Eu和Ti(圖4b),富 Ga,10000Ga/Al值為3.0～4.1,接近全球A型花崗巖的平均值(～3.75)(Whalen et al.,1987),在 10000Ga/Al和(Na2O+K2O)/CaO 對(duì)(Zr+Nb+Ce+Y)的判別圖解里(圖5),淡竹流紋斑巖均落入 A型花崗巖區(qū)域,而與分異的 I或 S型花崗巖相區(qū)別。根據(jù)Watson and Harrison(1983)的公式,用巖石主量元素及其Zr含量計(jì)算得到流紋斑巖的鋯石飽和溫度在881～912 °C之間,顯示其接近于A型而明顯高于I和S型花崗巖的原始巖漿溫度。綜上所述,巖石學(xué)、地球化學(xué)特征及巖漿形成溫度都表明淡竹流紋斑巖屬于典型的A型流紋斑巖。

表3 淡竹流紋斑巖的主量元素(%)和微量元素組成(×10-6)Table3 Major (%) and trace element (×10-6) concentrations of the rhyolitic porphyries in the Danzhu region

圖4 淡竹流紋斑巖的稀土元素球粒隕石標(biāo)準(zhǔn)化配分圖解(a)和原始地幔標(biāo)準(zhǔn)化的微量元素圖解(b)(球粒隕石與原始地幔值取自Sun and McDonough,1989)Fig.4 Chondrite-normalized REE patterns(a) and primitive mantle-normalized spidergrams(b) of the Danzhu rhyolite porphyries(chondrite and primitive mantle values are from Sun and McDonough (1989))

表4 淡竹流紋斑巖的鋯石Hf同位素組成Table4 LA-MC-ICP-MS zircon Lu-Hf isotope results for the rhyolitic porphyries in the Danzhu region

已有研究表明,A型花崗巖(流紋巖)的成因有如下幾種可能:(1)幔源玄武質(zhì)巖漿的結(jié)晶分異作用(Mushkin et al.,2003);(2)深部地殼巖石的部分熔融(Clemens et al.,1986;Whalen et al.,1987;King et al.,1997;Wu et al.,2002);(3)殼幔巖漿混合(Yang et al.,2006)。首先,研究區(qū)和相鄰地區(qū)缺乏同時(shí)代的大規(guī)?；曰鸪蓭r這一事實(shí),可以排除本區(qū)流紋斑巖由基性巖漿分離結(jié)晶作用所成的可能性。此外,A型花崗巖(流紋巖)具有富堿、貧水的晶體粥特性,說(shuō)明它們也不可能通過(guò)較大規(guī)模的分離結(jié)晶作用所形成(吳福元等,2007)。另外,淡竹流紋斑巖中尚未發(fā)現(xiàn)有中基性的暗色包體,也可以排除巖漿混合的成因模式。那么,深部陸殼巖石部分熔融可能是形成淡竹流紋斑巖較理想的成巖模式。流紋斑巖具有明顯的Pb的正異常和Nb的負(fù)異常,也表明巖石熔融的源巖可能以陸殼物質(zhì)為主。顯著的Sr及Eu的負(fù)異常,則進(jìn)一步說(shuō)明長(zhǎng)英質(zhì)巖石在其源區(qū)可能占主導(dǎo)(吳福元等,2007)。此外,流紋斑巖的兩階段Hf模式年齡t2DM(2.46～2.76 Ga)為新太古代,在εHf(t)-t圖解上,其落入華夏陸塊古老基底(八都群)演化域內(nèi)(圖6)。因此,華夏陸塊深部古老基底巖石(類似八都群)的部分熔融可能是淡竹流紋斑巖的巖漿物質(zhì)的主要來(lái)源。

4.2 構(gòu)造意義

眾所周知,一系列重要的地質(zhì)事件發(fā)生于古元古代已經(jīng)成為共識(shí),如全球性的碰撞造山事件和Columbia超級(jí)大陸的形成(Rogers and Santosh,2002;Zhao et al.,2002,2004,2009)、大陸地殼的快速生長(zhǎng)(Condie,1998,2000)、超級(jí)地幔柱活動(dòng)(Condie et al.,2001)等。中國(guó)東部的華北陸塊(Zhao et al.,2002,2004,2009;Wang et al.,2007;Hou et al.,2008;Peng et al.,2008;He et al.,2009)和揚(yáng)子陸塊(Zhang et al.,2006;Sun et al.,2008;Wu et al.,2008;Zhao et al.,2010)同樣有著與 Columbia超大陸聚合和裂解有關(guān)的古元古代大規(guī)模構(gòu)造-巖漿作用的響應(yīng)。在華夏陸塊,越來(lái)越多古元古代構(gòu)造-熱事件近年來(lái)陸續(xù)被發(fā)現(xiàn)(Li,1997;Li et al.,2000;Xiang et al.,2008;Liu et al.,2009;Xia et al.,2012;Yu et al.,2009,2012;Chen and Xing,in press),但目前對(duì)這些地質(zhì)事件性質(zhì)還存在認(rèn)識(shí)上的分歧。如,基于對(duì)武夷地塊古元古代的S型(1888～1875 Ma)和A型(1867～1855 Ma)花崗巖及裂谷型基性巖(天井坪組)(1760 Ma)的研究(Li,1997;Li et al.,2000),Yu et al.(2009)提出華夏陸塊可能存在一個(gè)與Columbia超大陸匯聚有關(guān)的造山旋回:同造山-后造山期(1.89～1.83 Ga)和非造山(裂谷)期(1.80～1.76 Ga);然而,Xiang et al.(2008)強(qiáng)調(diào)板內(nèi)基性-超基性的巖漿活動(dòng)早在～1.85 Ga就已開始;Xia et al.(2012)則認(rèn)為武夷地塊北部古元古代A和S型花崗巖(1.89～1.85 Ga)同樣形成于板內(nèi)裂谷環(huán)境;最近,在武夷地塊中部還發(fā)現(xiàn)有～1.84 Ga過(guò)鋁質(zhì) S型花崗巖,其成因機(jī)制也與板內(nèi)伸展環(huán)境有關(guān)(Chen and Xing,in press)。我們注意到,武夷地塊出露的古元古代 S和A型花崗巖(流紋斑巖),其實(shí)是在相隔很短的時(shí)間內(nèi)(1.89～1.83 Ga)形成,暗示它們可能是同一構(gòu)造-熱事件背景下的產(chǎn)物。

圖5 淡竹流紋斑巖的(Na2O+K2O)/CaO-(Zr+Nb+Ce+Y)圖解(a)和(Zr+Nb+Ce+Y)-10000×Ga/Al圖解(b) (底圖據(jù)Whalen et al.,1987)Fig.5 (Na2O+K2O)/CaO vs (Zr+Nb+Ce+Y) (a) and (Zr+Nb+Ce+Y) vs 10000×Ga/Al (b) plots for the Danzhu rhyolitic porphyries (after Whalen et al.,1987)

圖6 淡竹流紋斑巖的鋯石εHf(t)-t圖解(八都群、古元古代S和A型花崗巖的鋯石Hf同位素?cái)?shù)據(jù)來(lái)自Liu et al.(2009),Yu et al.(2009,2012)和Xia et al.(2012))Fig.6 εHf(t) vs 207Pb/206Pb age plot for the Danzhu rhyolitic porphyries(The zircon Hf-isotope data of the Badu Group and Paleoproterozoic S- and A-type granites are from Liu et al.,(2009);Yu et al.,(2009,2012);Xia et al.,(2012))

武夷地塊～1.82 Ga的A型流紋斑巖的發(fā)現(xiàn)和確證,為深入理解華夏陸塊的基底性質(zhì)和早期構(gòu)造格局的演化提供了新的地質(zhì)內(nèi)容,同時(shí)表明武夷地塊的S和A型花崗巖(流紋斑巖)的成巖年齡并無(wú)明顯的規(guī)律性。另外,武夷地塊的A型花崗巖(流紋斑巖)形成于1.87～1.82 Ga,至少還說(shuō)明～1.87 Ga之后區(qū)域上一直處于板內(nèi)伸展環(huán)境。一般認(rèn)為,花崗巖(流紋巖)的地球化學(xué)成分取決于其源巖的礦物組成和化學(xué)成分、熔融時(shí)的物理化學(xué)條件(包括溫度、壓力和揮發(fā)份)和其后巖漿的演化(如分離結(jié)晶作用、巖漿混合作用、同化混染作用等),因此大部分花崗巖(流紋巖)的地球化學(xué)組成與構(gòu)造背景可能無(wú)直接的對(duì)應(yīng)關(guān)系(吳福元等,2007)。但是需要指出的是,學(xué)術(shù)界對(duì)A型花崗巖(流紋巖)的構(gòu)造指示意義還是有較統(tǒng)一的認(rèn)識(shí),認(rèn)為它們的產(chǎn)出通常與伸展構(gòu)造環(huán)境有關(guān)(Whalen et al.,1987;Clemens et al.,1986;Barbarin,1999)。因此,結(jié)合區(qū)域地質(zhì)資料,我們傾向于認(rèn)為北武夷淡竹A型流紋斑巖可能是板內(nèi)伸展背景下巖漿活動(dòng)的產(chǎn)物,也正是由于這種拉張作用的機(jī)制,促使了軟流圈地幔上涌、巖石圈地幔減壓熔融生成基性巖漿底侵,導(dǎo)致地殼深熔作用形成了中-北武夷地區(qū)的古元古代花崗巖(流紋斑巖)。如前所述,流紋斑巖具有較高的形成溫度和負(fù)的εHf(t)值,表明淡竹流紋斑巖的成巖過(guò)程中幔源巖漿可能僅提供了熱量而缺乏物質(zhì)的輸入。另外,區(qū)域上未發(fā)現(xiàn)有大規(guī)模同期的雙峰式火山巖,也暗示這種伸展環(huán)境可能僅處于板內(nèi)拉張作用的初級(jí)階段,尚未進(jìn)入板內(nèi)裂谷期。

淡竹流紋斑巖中印支期變質(zhì)年齡信息的發(fā)現(xiàn)和厘定,還說(shuō)明它與區(qū)域上出露的其它古元古代 S和A型花崗巖一樣,共同經(jīng)歷了一次變質(zhì)事件的改造(Xiang et al.,2008;Yu et al.,2009;Xia et al.,2012)。事實(shí)上,華南及其鄰區(qū)在二疊紀(jì)-三疊紀(jì)時(shí)處于地質(zhì)構(gòu)造的活躍期,如 Sibumasu(中緬?cǎi)R蘇)地塊向印支-華南板塊增生(Carter et al.,2001)、印支板塊與華南板塊開始碰撞(Lepvrier et al.,2004)以及華南板塊與華北板塊的碰撞等(Ames et al.,1993;Li et al.,1993;Jahn,1998;Zheng et al.,2006)。最近,Li and Li(2007)提出華夏陸塊晚二疊世-三疊紀(jì)(251～233 Ma)強(qiáng)烈的板內(nèi)造山作用,可能還與中生代太平洋板塊平俯沖到華南板塊有關(guān)。盡管,目前對(duì)華南印支造山事件的認(rèn)識(shí)還存在分歧,但是都認(rèn)同這期重要的造山作用直接導(dǎo)致了中-北武夷地區(qū)基底巖石發(fā)生了強(qiáng)烈的變質(zhì)變形作用(達(dá)角閃巖相)。同時(shí),也造成包括淡竹流紋斑巖在內(nèi)的大部分古元古代花崗巖中的鋯石發(fā)生了不同程度的Pb丟失,并促使這些鋯石發(fā)生不同程度的重結(jié)晶作用或形成新生的變質(zhì)鋯石(或增生邊)(Yu et al.,2009,2012;Xia et al.,2012)。

5 結(jié) 論

(1) 獲得華夏陸塊北武夷淡竹流紋斑巖的LA-ICP-MS 鋯石U-Pb年齡為1819±16 Ma,為古元古代巖漿活動(dòng)的產(chǎn)物,并受到印支期一定的變質(zhì)改造作用影響。

(2) 元素地球化學(xué)組成和鋯石Hf同位素特征表明,～1.82 Ga的淡竹流紋斑巖具有典型的A型花崗巖(流紋巖)的特征,來(lái)自華夏陸塊基底新太古代陸殼物質(zhì)的再造重熔,形成于板內(nèi)伸展環(huán)境。

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