聶峰　石永紅　王娟　康濤　曹晟

合肥工業(yè)大學(xué)資源與環(huán)境工程學(xué)院，合肥　230009

磁鐵石榴角閃巖；等壓降溫；鋯石U-Pb年齡；郯廬斷裂

1　引言

圖1　研究區(qū)地質(zhì)簡圖(a)-肥東地區(qū)(安徽段)地質(zhì)圖；(b)-地質(zhì)剖面圖Fig.1　Simplified geological sketch map of the study area(a)-geological sketch map of the Feidong area (Anhui segment); (b)-geological profile

長期以來，郯廬斷裂的形成機(jī)制和演化過程一直是研究的熱點(diǎn)，特別是該斷裂與大別-蘇魯造山帶在時空上、成因上的聯(lián)系一直為人們所關(guān)注(Xuetal., 1987; Zhuetal., 2005, 2009, 2010)。目前，主流的觀點(diǎn)認(rèn)為郯廬斷裂形成于大別-蘇魯造山帶碰撞俯沖過程中(Zhangetal., 1984; Hsuetal., 1987; Watsonetal., 1987; Yin and Nie, 1993; Li, 1994; 萬天豐和朱鴻, 1996; Chang, 1996; 王小鳳等, 1998; Chung, 1999; Zhuetal., 2005, 2009, 2010)。另一種觀點(diǎn)則強(qiáng)調(diào)郯廬斷裂形成于該造山帶折返之后(Xuetal., 1987; Okay and Sengor, 1992; Xu and Zhu, 1994; Wangetal., 2003; Mengetal., 2007)。近年來對郯廬斷裂的研究多集中于年代學(xué)和構(gòu)造地質(zhì)學(xué)方面(Xuetal., 1980, 1987, 1994; Lin and Fuller, 1990; Okay and Sengor, 1992; Yin and Nie, 1993; Li, 1994; Linetal., 2005, 2009; Lin and Li, 1995; Zhang, 1997; Gilderetal., 1999; Schmidetal., 1999; Wangetal., 2003; Mengetal., 2007; Zhuetal., 2005, 2009, 2010; 朱光等, 1998, 2001, 2002, 2003, 2005a, b, 2006a, b, 2009a; 王勇生等, 2004, 2005a, b, 2006; 牛漫蘭等, 2002, 2005, 2006; Zhang and Teyssier, 2013; Zhangetal., 2013; 趙田等, 2014)，多關(guān)注其不同層次的變形特征和水平錯斷距離(Zhuetal., 2005, 2009, 2010)。而對于郯廬主干斷裂(安徽段)空間上的展布狀況仍缺乏統(tǒng)一的認(rèn)識。Xuetal. (1987)、Zhangetal. (2007, 2013a, b)、Zhuetal. (2005, 2009, 2010) 認(rèn)為郯廬主干斷裂位于合肥盆地東緣，即解集-闞集-王鐵-橋頭集以東的張八嶺隆起區(qū)。但是，趙田等(2014)則認(rèn)為該主干斷裂處于合肥盆地內(nèi)部(圖1)。關(guān)于該斷裂帶的橫向展布，多數(shù)學(xué)者認(rèn)為其沿著一個狹窄的線性條帶分布，并僅限于揚(yáng)子地塊上。上述關(guān)于郯廬斷裂帶空間位置的不同認(rèn)識，可能是由于對張八嶺隆起區(qū)巖石單元構(gòu)成缺乏深入的解剖所致，特別是相關(guān)精細(xì)的變質(zhì)巖石學(xué)研究的不足，影響了人們對郯廬斷裂帶的全面深入理解。作為切穿了華北和揚(yáng)子兩大板塊的郯廬斷裂帶，其展布范圍可能不僅局限于揚(yáng)子板塊上，橫向上的范圍應(yīng)當(dāng)較為寬闊，從而部分進(jìn)入華北板塊內(nèi)。若如此，在走滑斷裂作用下，該帶內(nèi)可能有來自兩側(cè)板塊的巖石。為此，本文對張八嶺隆起區(qū)肥東群的特征性巖石——磁鐵石榴角閃巖進(jìn)行了詳細(xì)變質(zhì)巖石學(xué)、年代學(xué)研究。研究顯示，該類巖石可能來源于華北板塊，推測其可能因郯廬斷裂的走滑作用被卷入該帶內(nèi)。

2　研究剖面及樣品介紹

本次研究區(qū)域位于張八嶺隆起區(qū)南段肥東地塊最南緣。該地塊西側(cè)為合肥盆地，東側(cè)為全椒盆地所覆蓋(圖1)。根據(jù)Zhuetal. (2005, 2009, 2010)和康濤等(2013)的研究，認(rèn)為該地塊自西向東可分為3個單元：(1)單元-I：主體為花崗片麻巖，沿浮渣山-火龍山-尖山-后分河一線分布，其間夾有斜長角閃巖和黑云斜長片麻巖透鏡體；(2)單元-II：主要為黑云斜長片麻巖，分布于太子山一線；(3)單元-III：主要由角閃斜長片麻巖、含磷大理巖、斜長角閃巖、石榴黑云母片麻巖構(gòu)成，沿龍山-方集-上份葉-蠻山口分布。該地塊中發(fā)育有3條NNE向韌性剪切帶，每條寬度在100～200m(圖1a)，均表現(xiàn)為左旋走滑運(yùn)動特征(Zhuetal., 2005, 2009, 2010)。

重點(diǎn)研究的剖面位于單元-III東南緣，蠻山口北東，上份葉東南，坐標(biāo)為：緯度31°46.148′和經(jīng)度117°36.663′，剖面總長約200m。在該剖面上共采集23塊樣品，其中變形花崗巖5塊，斜長角閃巖3塊，石榴黑云片巖7塊，磁鐵石榴角閃巖8塊，其中用于此次研究分析的共計4塊(圖1b)，分別為QT026-1、QT023-1、TF003-1和TF001-2。研究剖面為NW-SE向，由西向東出露的巖性依次為變形花崗巖、斜長角閃巖、石榴黑云片巖、磁鐵石榴角閃巖、石榴黑云片巖和斜長角閃巖(圖1b、圖2a)。這些巖石均以單斜層形式產(chǎn)出，面理傾向為134°～166°、傾角47°～65°。其中磁鐵石榴角閃巖夾持于石榴黑云片巖之中，其出露的寬度約40～60m，斜長角閃巖和石榴黑云片巖寬度多在20～30m(圖2a)，這三類巖石緊密共生，沿走向延伸<300m，表現(xiàn)為構(gòu)造透鏡體形式。而變形花崗巖則沿走向穩(wěn)定延伸，并與斜長角閃巖順層接觸，露頭規(guī)模可見到微褶和長石拉長變形之特征(圖2a)。

表1磁鐵石榴角閃巖代表性礦物成分(wt%)

Table 1The representative mineral compositions (wt%) of magnet garnet amphibole

礦物包體基質(zhì)1基質(zhì)2GrtGruFe-HblGrtGruFe-HblGrtGruFe-HblSiO236.4747.9442.6436.4850.4841.2836.1750.3240.76TiO20.000.000.000.000.020.030.000.000.13Al2O319.920.4410.9320.251.1212.8020.010.9613.60FeO33.0237.8927.1733.6839.0828.1833.8139.4728.14Cr2O30.000.000.000.010.000.000.030.000.00MnO1.180.160.051.140.140.051.080.150.05MgO0.776.573.760.685.412.640.895.472.36CaO8.090.6211.457.251.2711.517.040.9911.78Na2O0.030.311.440.030.271.560.040.211.64K2O0.000.010.090.000.010.110.000.010.10Total99.4793.9497.5499.5397.7998.1699.0697.5798.55O12.0023.0023.0012.0023.0023.0012.0023.0023.00Si2.967.926.612.977.986.392.967.996.31Al1.910.092.001.940.212.341.930.182.48Fe3+0.160.000.540.120.000.560.160.000.45Ti0.000.000.000.000.000.000.000.000.02Cr0.000.000.000.000.000.000.000.000.00Fe2+2.085.232.992.175.173.092.155.243.20Mn0.080.020.010.080.020.010.080.020.01Mg0.091.620.870.081.280.610.111.300.54Ca0.710.111.900.630.221.910.620.171.96Na0.010.100.430.000.080.470.010.060.49K0.000.000.020.000.000.020.000.000.02Sum8.0015.0915.358.0014.9515.408.0014.9515.47

3　巖相學(xué)和主要礦物化學(xué)分析

由于此次研究的重點(diǎn)是磁鐵石榴角閃巖，故礦物化學(xué)分析主要是針對該類巖石進(jìn)行。礦物成分測試由合肥工業(yè)大學(xué)資源與環(huán)境工程學(xué)院電子探針實(shí)驗室完成，儀器型號為JEOL JAX-8230，實(shí)驗條件為：加速電壓15kv，電子束流20nA，電子束斑為3μm。其中石榴石和鐵閃石、鐵普通角閃石結(jié)構(gòu)式分別以12和23個O進(jìn)行計算，F(xiàn)e2+的校正則分別以電價平衡法(Droop, 1987)、全Fe2+和Si+Al+Ti+Mg+Fe+Mn=13進(jìn)行估算。代表性礦物分析結(jié)果見表1。此外，文中礦物縮寫據(jù) Whitney and Evans (2010)：Grt=石榴石；Gru=鐵閃石；Fe-Hbl=鐵普通角閃石；Fe-Ts=鐵契爾馬克閃石；Qz=石英；Amp=角閃石；PL=斜長石；Kfs=鉀長石；Bt=黑云母；Ms=白云母；Mag=磁鐵礦；Chl=綠泥石；Ap=磷灰石。

圖2　研究區(qū)巖石野外照片和顯微照片(a)-主要巖性接觸關(guān)系；(b)-變形花崗巖顯微照片；(c)-斜長角閃巖顯微照片；(d)-石榴黑云片巖顯微照片；(e、f)-磁鐵石榴角閃巖顯微照片F(xiàn)ig.2　The field photos and microstructure photos in the study area(a)-contact relation of main lithology; (b)-microstructure photos of deformational granites; (c)-microstructure photos of Plagioclase amphibolite; (d)-microstructure photos of garnet biotite schist; (e, f)-microstructure photos of magnet garnet amphibole

3.1　巖相學(xué)分析

(1)變形花崗巖(樣品QT026-5)：主要為鉀長石(20%～25%)+斜長石(20%)+石英(30%)+黑云母(10%)+白云母(10%～15%)(圖2b)。這些礦物因剪切變形作用，多表現(xiàn)為拉長變形之特征，粒徑大小不一。其中鉀長石粒徑為0.2～0.5mm，斜長石粒徑為0.2～0.5mm，石英粒徑為0.2～0.5mm；黑云母則表現(xiàn)為他形-半自形，粒徑約0.2～0.3mm。白云母則多為多硅白云母，Si4+多在3.08～3.12。

(2)斜長角閃巖(樣品QT023-1)：組成礦物為角閃石(50%～55%)+斜長石(20%～25%)+石英(10%～15%)+黑云母(10%～15%)+磷灰石(5%)(圖2c)。角閃石呈自形-半自形，鱗片狀定向排列，粒徑0.2～2mm，顆粒邊緣發(fā)生輕微綠泥石化；斜長石為半自形-他形結(jié)構(gòu)，粒徑為0.1～0.3mm；黑云母半自形-他形，粒徑大小～0.2mm，部分顆粒已完全退變?yōu)榫G泥石，僅保留黑云母結(jié)構(gòu)假象；石英他形結(jié)構(gòu)，粒徑大小0.2～0.5mm；磷灰石他形結(jié)構(gòu)，顆粒大小為0.1～0.3mm。

圖3　磁鐵石榴角閃巖的石榴石X-ray mapping圖、角閃石BSE圖及成分剖面圖(a)-石榴石X-ray mapping圖；(b)-石榴石成分剖面圖；(c)-鐵閃石BSE圖；(d)-鐵閃石成分剖面圖；(e)-鐵普通角閃石BSE圖；(f)-鐵普通角閃石成分剖面圖Fig.3　X-ray mapping image of garnet, BSE images of amphibole and compositional profiles from magnet garnet amphibole(a)-X-ray mapping image of garnet; (b)-compositional profiles of garnet; (c)-BSE image of grunerite; (d)-compositional profiles of grunerite; (e)-BSE image of Fe-hornblende; (f)-compositional profiles of Fe-hornblende

(3)石榴黑云母片巖(樣品TF003-1)：礦物組成為石榴石(30%)+黑云母(40%)+石英(20%)+白云母(10%)+磁鐵礦(5%)+磷灰石(3%)(圖2d)，石榴石呈自形-半自形，粒徑1～3mm，內(nèi)含大量石英、云母、磷灰石包體，包體主要分布于核部，邊部無包體；黑云母為自形-半自形結(jié)構(gòu)，粒徑大小0.2～1mm。白云母為自形-半自形，粒徑0.2～0.5mm，夾持于黑云母之間；石英呈他形，粒徑0.2～3mm；磁鐵礦呈自形，粒徑0.2～0.3mm。

(4)磁鐵石榴角閃巖(樣品TF001-2)：該類巖石是本次分析的重點(diǎn)巖石，其礦物組合為石榴石(10%～15%)+鐵普通角閃石(20%～25%)+鐵閃石(25%～30%)+磁鐵礦(25%～30%)+石英(10%～15%)+磷灰石(1%～3%)(圖2e, f)。石榴石呈半自形-他形，粒徑約為0.5～5mm，內(nèi)部包含鐵閃石、鐵普通角閃石、磁鐵礦、石英包體(圖2e, f)，其多破裂，裂隙之中常常被綠泥石所充填；鐵閃石以基質(zhì)和包體兩種形式存在，其中基質(zhì)中的的鐵閃石呈他形，粒徑0.1～0.5mm；鐵普通角閃同樣以包體和基質(zhì)兩種形式存在，基質(zhì)中的鐵普通角閃石為他形，粒徑0.1～1mm，其邊緣或裂隙常常被綠泥石所替代；石英為他形，粒徑0.1～2mm；磁鐵礦呈自形-他形，粒徑0.1～3mm；磷灰石為他形，粒徑0.1～0.2mm。

圖4　礦物成分圖(a)-石榴石成分三角圖；(b)-鐵閃石成分圖；(c)-鐵普通角閃石成分圖Fig.4　Compositional diagrams of the metamorphic minerals(a)-compositional triangle of garnet; (b)-compositional diagrams of grunerite; (c)-compositional diagrams of Fe-hornblende

3.2　磁鐵石榴角閃巖主要礦物化學(xué)分析

為了較為精確地揭示該類巖石中主要礦物成分變化特征，以及準(zhǔn)確評價其P-T演化過程，本次研究對其中的石榴石、角閃石進(jìn)行了細(xì)致的成分剖面研究。研究顯示：

(1)石榴石：在X-ray Mapping 圖中(圖3a)，其無明顯的環(huán)帶特征，顏色較為均勻。但在成分剖面中(圖3b)，石榴石顯示了輕微的核、邊結(jié)構(gòu)。自核部至邊部，鐵鋁榴石組分逐漸增高(64%核→73%邊)，鎂鋁榴石組分輕微增高(3%核→5%邊)，鈣鋁榴石組分則略微降低(15%核→11%邊)，錳鋁榴石組分較為平坦。同樣地，在成分三角圖中(圖4a)，核、邊成分也略顯差異，其中鐵鋁榴石組分逐漸增高，鎂鋁榴石組分和鈣鋁榴石組分相對較低。但就總體而言，石榴石成分變化似乎并不顯著，這意味著其可能處于快速生長條件下，并已達(dá)到均一化程度。

(2)角閃石：該類巖石具有鐵閃石和鐵普通角閃石兩種閃石，且兩者都以包體和基質(zhì)形式存在(圖2e)。對基質(zhì)中的鐵閃石和鐵普通角閃石的定量分析顯示，無論是在礦物顏色方面，還是礦物成分方面，這兩個閃石均較為均勻，沒有明顯的環(huán)帶特征(圖3c-f)。其中鐵閃石中的Fe2+、Mg2+和Al3+成分十分平坦，沒有早期或后期變質(zhì)作用的疊加。稍有不同的是鐵普通角閃石Al3+自核部至邊部輕微降低(2.5核部→2.0邊部)。參照Hammarstrometal. (1986)、Hollisteretal. (1987)、Johnsonetal. (1989)和Schmidt. (1992) 的研究，我們推測這也許反映的是一種降壓的過程。然而，從成分剖面看(圖3f)，這種退變降壓可能并不顯著。此外，以基質(zhì)和包體形式存在的兩種閃石的成分也沒有顯著的成分差異(圖4b, c)。其中基質(zhì)和包體的鐵閃石成分變化基本位于同一個區(qū)域(圖4b)，略有差異的是包體中的Mg/(Fe2++Mg)比值略高于基質(zhì)的Mg/(Fe2++Mg)。對于包體中的鐵普通角閃石其嚴(yán)格位于Fe-Hbl區(qū)域，而基質(zhì)中鐵普通角閃石主體位于Fe-Hbl區(qū)域，部分落入Fe-Ts區(qū)域(圖4c)。

4　磁鐵石榴角閃巖峰期變質(zhì)P-T條件評價

依據(jù)巖相學(xué)和礦物化學(xué)分析(圖2e, f、圖3)，大致可以判定磁鐵石榴角閃巖可能具有兩個階段的變質(zhì)礦物組合：①階段：Grt-I(核部)+Gru(包體)+Fe-Hbl(包體)+Qtz(包體)+Mag(包體)；②階段：Grt-II(邊部)+Gru(基質(zhì))+Fe-Hbl(基質(zhì))+Qtz(基質(zhì))+Mag(基質(zhì))+Ap磷灰石(基質(zhì))。據(jù)此，本文對其進(jìn)行了細(xì)致的溫壓評價。在成分選取方面，參照成分剖面的分析(圖2e, f、圖3)，①階段變質(zhì)的成分選取石榴石核部，以及石榴石包體中的鐵閃石、鐵普通角閃石進(jìn)行計算；②階段變質(zhì)的成分選取石榴石邊部，基質(zhì)中的鐵閃石、鐵普通角閃石核部或近邊部的成分進(jìn)行計算。同時，為保證分析計算的統(tǒng)計意義，本次研究在基質(zhì)中共選取12個礦物對，包體中選取6個礦物對進(jìn)行溫壓估算?；赪orley and Powell (2000)、Powell and Holland (2008)和魏春景等 (2009) 的研究，此次P-T條件的評價選用了Thermocalc version 3.33 (Holland and Powell, 1998) 平均溫壓法(av-PT)進(jìn)行，計算結(jié)果見表2。由于①和②階段的礦物組合基本類似，因此，這兩個階段的PT值基本是由6條獨(dú)立反應(yīng)線限定：

表2磁鐵石榴角閃巖P(GPa)-T(℃)條件

Table 2TheP-Tconditions for magnet garnet amphibole

樣品TPσTσPσfit包體17000.662490.673.2226880.672150.592.6636520.661790.532.4946440.842080.602.8956180.791620.472.2666160.851680.462.06基質(zhì)76030.941780.442.1085970.631960.802.3196110.791310.361.67106300.591680.482.00116150.691510.411.82126140.801470.391.73136110.771460.391.78146370.841490.381.80156320.711440.401.78166430.511470.431.71176320.531470.421.64186120.941520.711.74

圖5　磁鐵石榴角閃巖峰期變質(zhì)P-T條件圖Fig.5　Plot of metamorphic peak P-T conditions of the magnet garnet amphibole

1) 10py+21fact=10gr+15grun+6tr

2) 4parg+12q=2gr+cumm+ts+2gl

3) 42parg+114q=14py+28gr+9cumm+21gl+12H2O

4) 7alm+12parg+36q=7py+6gr+3grun+3ts+6gl

5) 12parg+36q=2py+4gr+3tr+3ts+6gl

6) 3py+14gr+3grun+12q+12mt=11alm+12andr+3ts

對比石永紅等(2009)對肥東群的研究，本次磁鐵石榴角閃巖的變質(zhì)條件明顯高于他們的結(jié)果，但接近于王娟等(2014)確定的華北板塊的東部陸塊中的五河群變質(zhì)條件。結(jié)合確定的變質(zhì)年齡2450～2490Ma(見后述)和等壓降溫P-T樣式分析(圖5、圖6g)，該類巖石變質(zhì)特征十分類似Zhao and Zhai (2013)確定的2.5Ga變質(zhì)事件和演化過程。換言之，該巖石可能源于華北板塊。

5　鋯石U-Pb定年

本次對磁鐵石榴角閃巖(樣品TF001-2)進(jìn)行了鋯石U-Pb定年。鋯石單礦物挑選工作由河北省地勘局廊坊實(shí)驗室完成，樣品重約15kg，共挑選出約200單顆鋯石。鋯石制靶由合肥工業(yè)大學(xué)LA-ICPMS潔凈實(shí)驗室完成，鋯石陰極發(fā)光(CL)照相由桂林理工大學(xué)電子探針實(shí)驗室完成，儀器型號XM-Z09013TPCL。鋯石U-Pb定年分析由合肥工業(yè)大學(xué)LA-ICPMS實(shí)驗室完成，實(shí)驗條件：激光器工作頻率為10Hz，其中激光剝蝕束斑為32μm，信號有效采集時間為50s，每分析測試5個樣品點(diǎn)測兩次標(biāo)準(zhǔn)鋯石91500。鋯石數(shù)據(jù)處理采用ICPMSDateCal7.5軟件(Ludwing, 2003)和ISOPLOT程序，單個測點(diǎn)同位素年齡的誤差為1σ，加權(quán)平均年齡具有95%置信度。此外，鋯石中礦物包體的測定由中國科學(xué)技術(shù)大學(xué)地球和空間科學(xué)學(xué)院拉曼實(shí)驗室分析完成，儀器型Thermo Fisher DXR。年齡分析數(shù)據(jù)見表3。

根據(jù)顯微鏡下的透、反射光的研究，樣品TF001-2中的鋯石多為渾圓狀或短柱狀，自形程度較低，粒徑50～300μm，長寬比為2:1～1:1。CL圖像顯示，鋯石多為斑雜狀分帶、面狀結(jié)構(gòu)、云霧狀分帶，無震蕩環(huán)帶(圖6a-c)。此外，這些鋯石常具有較多的礦物包體，拉曼光譜分析顯示這些包體多為角閃石和磷灰石(圖6a-d)。

由于鋯石粒徑相對較小和激光剝蝕束斑較大的原因，本次鋯石U-Pb定年僅獲得了40個數(shù)據(jù)點(diǎn)，其中3個數(shù)據(jù)點(diǎn)為諧和年齡，其余數(shù)據(jù)點(diǎn)為不諧和年齡(表3)。3個諧和年齡的鋯石的Th/U比值分別為0.09、0.11和0.15，基本上小于或約等于0.1，年齡分別是2473±15Ma、2490±18Ma和2450±15Ma，加權(quán)平均年齡為2469±49Ma(MSWD=1.5，n=3)(圖6g)。而37個不諧和年齡的鋯石的Th/U比值0.1～0.4(多為0.1)，其年齡范圍為2350～2490Ma，均位于不協(xié)和線上，上交點(diǎn)年齡為2458±25Ma(圖6g)。結(jié)合CL圖像分析，這些鋯石應(yīng)均為變質(zhì)鋯石(吳元保和鄭永飛, 2004)，故它們的年齡代表的是變質(zhì)年齡。盡管，這里的上交點(diǎn)年齡并不能準(zhǔn)確再現(xiàn)該變質(zhì)事件的精確年齡，但其具有重要的參考價值。參照3個諧和年齡來看(圖6g)，上交點(diǎn)年齡與其十分類同，這意味著3個諧和年齡應(yīng)該能較為確切地反映某一變質(zhì)事件年齡。參照Gebaueretal. (1997)、Hermannetal. (2001)、吳元保和鄭永飛(2004)、Liuetal. (2004, 2011)、Liuetal. (2007) 和Zheng (2008)的研究，并根據(jù)此次拉曼光譜和定年分析結(jié)果，我們認(rèn)為這3個諧和年齡反映的是①和②階段變質(zhì)作用年齡，因為這3個年齡均是在含角閃石和磷灰石礦物包裹體附近區(qū)域獲得的(圖6a-d)，而這些包體在①和②階段變質(zhì)礦物組合均普遍發(fā)育。

表3樣品TF001-2鋯石U-Pb定年分析數(shù)據(jù)

Table 3The zircon U-Pb date of the sample TF001-2

測點(diǎn)號232Th(×10-6)238U(×10-6)ThU206Pb(×10-6)同位素年齡(Ma)同位素比值207Pb206Pb1σ207Pb235U1σ206Pb238U1σ207Pb206Pb1σ207Pb235U1σ206Pb238U1σ140.91980.206596.902414112261142093110.15610.00108.26550.13100.38350.0024210.41090.096151.822422162265142093100.15690.00148.30660.12580.38360.0021312.995.30.135644.662456172246142031130.15890.00178.13460.12640.37040.0028431.61590.199375.952439142266132077130.15840.00138.31800.11730.38020.0028519.81840.107283.342439132227131997100.15840.00127.96370.11670.36310.0022620.21280.157643.542358181936151563150.15100.00155.73390.09640.27440.0029757.12630.2169112.1239810214210187970.15470.00097.24760.08410.33830.0014865.52250.2912111.9243910229311212790.15830.00098.56540.10500.39080.0020935.11590.220863.91239416209013179190.15420.00146.83180.09750.32020.00191037.31960.190383.82240511217113192580.15530.00117.47980.10450.34790.00171152.62550.206092.762350121992131662100.15020.00106.10990.08870.29420.00191227.31970.138481.84235012211313187680.15020.00107.01570.10210.33780.00171327.21650.165073.722388172189151979100.15380.00167.63820.12990.35940.00221419.71410.139562.932381132190161987130.15310.00117.63970.13510.36110.00261565.42370.2763111.7241611222116201290.15630.00107.91000.14390.36620.00191613.71070.128948.572422152250192060200.15680.00148.16750.17150.37660.00441730.02240.134198.342388112195161991150.15370.00107.68290.13760.36180.00321825.82330.1104101.73236112216414195970.15140.00117.42660.11450.35510.00161950.53730.1352139.4241610205214170680.15630.00096.54820.10370.30300.00172025.42310.109990.4823892112106171823170.15380.00106.95810.13660.32680.00362110.11100.092555.392473152378172265140.16160.00149.40490.17180.42100.00322216.61160.143148.972403172156161902120.15510.00157.35870.13450.34320.00252324.12080.115590.2723698217114196580.15200.00127.48140.11740.35630.00162410.189.50.113446.272490182387162267120.16330.00179.49700.16720.42150.00272510.41110.093150.802453192271172070130.15980.00138.36580.15790.37870.00272652.92270.2325103.72406112209172001110.15520.00107.80290.14490.36390.00232712.11050.115746.892417-1842240222038260.15640.00158.07690.19880.37190.00562831.41590.197864.85237717210714183990.15280.00146.96390.11210.33010.00182911.597.10.118248.282484152350152193120.16270.00149.11850.15150.40520.00253013.01050.124750.502450182292162116160.15950.00178.55960.15460.38850.00343116.01170.137352.16238315220115200680.15320.00137.74210.12510.36510.00183242.82430.176392.772444122083151732120.15890.00126.78100.11220.30820.00253316.11090.148155.882450152361142258110.15930.00149.22890.13820.41940.00253423.11480.156369.232455152302162128200.16000.00148.64880.14990.39120.00443528.11910.147180.92238914216013193380.15290.00127.39220.10850.34970.0017361052690.3899126.12408202214142006100.15560.00137.85270.11890.36510.00213713.11020.129046.572410162257172085200.15580.00148.23670.15430.38190.00433813.91480.094051.302376172010171662120.15250.00156.24260.11960.29410.00233927.31890.144485.332398212199171987140.15470.00197.71640.14210.36100.00294030.11470.205166.402413122219152009140.15590.00117.88970.12980.36570.0030

注：測點(diǎn)21、24、30三組數(shù)據(jù)為鋯石諧和年齡數(shù)據(jù)

圖6　樣品TF001-2鋯石CL圖、包體拉曼光譜及U-Pb定年諧和圖Fig.6　CL image, Raman spectra and concordia plots of zircons from sample TF001-2

6　磁鐵石榴角閃巖構(gòu)造歸屬討論

目前，人們普遍認(rèn)同郯廬斷裂(安徽段)是華北和揚(yáng)子板塊的重要邊界斷裂，其左旋走滑錯距長達(dá)>500km。主斷裂嚴(yán)格地呈狹窄的線性條帶發(fā)育于揚(yáng)子板塊內(nèi)(Xuetal., 1987; Zhuetal., 2005, 2009, 2010; Zhangetal., 2007, 2013a, b; 朱光等, 2009; 趙田等, 2014)，出露的寬度～5km(圖1)，而華北板塊并未受到郯廬斷裂明顯的改造和影響。換言之，郯廬斷裂帶內(nèi)的物質(zhì)均由揚(yáng)子板塊構(gòu)成，沒有任何源于華北板塊的物質(zhì)。

通常認(rèn)為華北和揚(yáng)子板塊的物質(zhì)差異主要體現(xiàn)在年齡方面。Zhao and Zhai (2013) 的研究表明，華北板塊最終拼合在1.85Ga完成，其內(nèi)部出露有極少量的始太古(3.8～3.6Ga)巖石地塊和部分2.8～2.7Ga初生陸殼。并強(qiáng)調(diào)華北板塊主體是由2.6～2.5Ga高級片麻巖和低-中級花崗巖-綠巖構(gòu)成，同時指出在～2.5Ga時期，華北東、西部陸塊普遍經(jīng)歷了綠片巖相至麻粒巖相的逆時針等壓降溫變質(zhì)作用，反映了源于地幔巖漿的底侵作用過程(Zhaoetal., 1998, 1999a, b; Zhao and zhai, 2013)。而1.95～1.85Ga變質(zhì)年齡則反映了華北板塊上西部、中部和東部陸塊的彼此之間碰撞俯沖時限。具體到郯廬斷裂西側(cè)的華北東部陸塊(圖1中的插圖)，Zhao and Zhai (2013) 認(rèn)為該陸塊在2.2～1.9Ga形成陸內(nèi)裂谷(張秋生, 1988; Lietal., 2004, 2005, 2006)，形成Longgang和Nangrim 2個塊體，并于1.9Ga時間俯沖碰撞拼合完成。相比較而言，揚(yáng)子板塊的巖石年齡具有較廣的年齡范圍，根據(jù)前人的研究其大致可分為7個年齡區(qū)間3.3～2.9Ga、2.7～2.5Ga、2.0～1.7Ga、～1.0Ga、850～650Ma、480～400Ma和245～200Ma(Hackeretal., 1998, 2006; Qiuetal., 2000; Ratschbacheretal., 2003, 2006; Wuetal., 2004; Zhengetal., 2007, 2008; Chenetal., 2003, 2009; Zhangetal., 2006; 鄭永飛和張少兵, 2007; Gaoetal., 2011; 魏君奇等, 2012)，其中后三個年齡范圍在揚(yáng)子板塊內(nèi)普遍發(fā)育，而前三個較老的年齡在揚(yáng)子板塊上出露極少，且2.7～2.5Ga反映的是角閃巖相的變質(zhì)事件。

對比本次張八嶺隆起區(qū)的肥東群中的磁鐵石榴角閃巖的年代學(xué)和變質(zhì)巖石學(xué)的研究來看，磁鐵石榴角閃巖中的3顆變質(zhì)鋯石的諧和年齡在2450～2490Ma，加權(quán)平均年齡為2496±49Ma，其十分接近于華北板塊的～2.5Ga的變質(zhì)年齡。而其他37顆變質(zhì)鋯石的不諧和年齡變化也較為單一，基本上在2350～2484Ma范圍(表3)，這暗示了該巖石類型可能源于華北板塊。然而，根據(jù)Qiuetal. (2000)和魏君奇等(2012)的研究，該年齡也同樣落入揚(yáng)子板塊的2.7～2.5Ga的范圍，似乎該巖石具揚(yáng)子板塊的親緣性。對此，本文沒有較為明確的結(jié)論。這里也許可以假設(shè)兩種情形：(1)磁鐵石榴角閃巖屬于華北板塊物質(zhì)，由于郯廬斷裂的左旋走滑作用，其呈構(gòu)造透鏡體的形式被卷入郯廬斷裂帶內(nèi)；(2)該巖石本身就歸屬于揚(yáng)子板塊，可能是郯廬斷裂帶(安徽段)內(nèi)出露的最老的巖石。

然而，結(jié)合地質(zhì)背景、構(gòu)造地質(zhì)學(xué)和變質(zhì)巖石學(xué)的研究，本次研究更傾向于前一種認(rèn)識。主要理由是：(a)從地質(zhì)背景來看，磁鐵石榴角閃巖在華北東部陸塊(安徽段)的五河雜巖和霍邱群中廣泛發(fā)育該類巖石(安徽省地質(zhì)礦產(chǎn)局, 1987; 楊曉勇等, 2012)，而安徽境內(nèi)的揚(yáng)子板塊則沒有這類相關(guān)類型巖石的確證。這暗示了該類巖石可能的物源來自于華北板塊；(b)在年齡變化方面，磁鐵石榴角閃巖的年齡十分單一，基本在～2.5Ga時間段(表1)，缺乏任何類似于揚(yáng)子板塊的較為年輕的年齡(850～650Ma、480～400Ma和245～200Ma)。且該年齡十分接近郯廬斷裂西側(cè)華北東部陸塊的年齡(Liuetal., 2009; 許文良等, 2006; 楊曉勇等, 2012; 圖1中插圖)，這從另一個側(cè)面反映了該巖石具華北板塊親緣性；(c)從變質(zhì)P-T演化來看，磁鐵石榴角閃巖顯示了輕微的等壓降溫特征(圖5)，與Zhao and Zhai (2013)闡述的華北東、西部陸塊在～2.5Ga發(fā)生的源于地幔巖漿的底侵事件十分吻合。此外，該巖石的變質(zhì)P-T條件也十分類似于五河群中石榴角閃巖的變質(zhì)條件(王娟等, 2014)，明顯不同于肥東群的變質(zhì)條件(石永紅等, 2009)；(d)構(gòu)造地質(zhì)學(xué)分析顯示，磁鐵石榴角閃巖及其圍巖是呈構(gòu)造透鏡體產(chǎn)于變形的花崗巖之中，周邊被糜棱巖所限定(圖1b)。其面理和線理與郯廬斷裂帶中韌性剪切帶的面理和線理呈大角度相交，暗示了其可能為一個外來的構(gòu)造塊體，并非原地物質(zhì)?；诒敬窝芯康慕Y(jié)果，我們認(rèn)為張八嶺隆起區(qū)的肥東群中的磁鐵石榴角閃巖可能來源于華北板塊，由于郯廬斷裂的左旋走滑作用被構(gòu)造并置于揚(yáng)子板塊中。進(jìn)一步地，可以推測郯廬斷裂東側(cè)界限位于方集-蠻山口以東的全椒盆地中，結(jié)合趙田等(2014)研究可以看出，該斷裂在橫向上(安徽段)的寬度至少在10～15km范圍(圖1)。

致謝感謝朱光教授在本文撰寫過程中的支持與幫助；感謝吳春明教授和林偉研究員對本文的審閱及意見。

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Zhu G, Wang YS, Liu GS, Niu ML, Xie CL and Li CC. 2005.40Ar/39Ar dating of strike-slip motion on the Tan-Lu Fault Zone, East China. Journal of Structural Geology, 27(8): 1379-1398

Zhu G, Xie CL, Wang YS, Niu ML and Liu GS. 2005a. Characteristics of the Tan-Lu high-pressure strike-slip ductile shear zone and its40Ar/39Ar dating. Acta Petrologica Sinica, 21(6): 1687-1702 (in Chinese with English abstract)

Zhu G, Niu ML, Liu GS, Wang YS, Xie CL and Li CC. 2005b.40Ar/39Ar dating for the strike-slip movement on the Feidong part of the Tan-Lu fault belt. Acta Geologica Sinica, 79(3): 303-316 (in Chinese with English abstract)

Zhu G, Wang YS, Wang DX, Niu ML, Liu GS and Xie CL. 2006a. Constraints of foreland sedimentation and deformation on synorogenic motion of the Tan-Lu fault zone. Chinese Journal of Geology, 41(1): 102-121 (in Chinese with English abstract)

Zhu G, Xu YD, Liu GS, Wang YS and Xie CL. 2006b. Structural and deformational characteristics of strike-slippings along the middle-southern sector of the Tan-Lu fault zone. Chinese Journal of Geology, 41(2): 226-241 (in Chinese with English abstract)

Zhu G, Liu GS, Niu ML, Xie CL, Wang YS and Xiang BW. 2009. Syn-collisional transform faulting of the Tan-Lu Fault Zone, East China. International Journal of Earth Sciences, 98(1): 135-155

Zhu G, Zhang L, Xie CL, Niu ML and Wang YS. 2009. Geochronological constraints on tectonic evolution of the Tan-Lu fault zone. Chinese Journal of Geology, 44(4): 1327-1342 (in Chinese with English abstract)

Zhu G, Niu ML, Xie CL and Wang YS. 2010. Sinistral to normal faulting along the Tan-Lu Fault Zone: Evidence for geodynamic switching of the East China continental margin. The Journal of Geology, 118(3): 277-293

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