大別山北麓早白堊世商城和達(dá)權(quán)店巖體的地球化學(xué)特征與成因

高昕宇, 趙太平, 施小斌, 張忠慧, 包志偉

(1. 中國科學(xué)院 南海海洋研究所 邊緣海地質(zhì)重點實驗室, 廣東 廣州 510301; 2. 中國科學(xué)院 廣州地球化學(xué)研究所礦物學(xué)與成礦學(xué)重點實驗室, 廣東 廣州 510640; 3. 河南省 山水地質(zhì)旅游公司，河南 鄭州 450001)

0 引 言

秦嶺-大別造山帶位于中國中部, 是一個經(jīng)歷了多階段構(gòu)造演化的復(fù)雜的大陸碰撞造山帶[1–2], 歷經(jīng)中元古代-古生代的多次陸緣裂解、增生和碰撞,三疊紀(jì)華北與揚子大陸板塊之間的對接, 三疊紀(jì)-侏羅紀(jì)的地殼縮短(拆離、推覆、A型俯沖), 東段大別造山帶的超高壓變質(zhì)作用[3–6], 以及隨后的強烈伸展塌陷和巖石圈拆離[7–10]。

秦嶺-大別造山帶演化后期廣泛發(fā)育花崗質(zhì)巖漿作用, 根據(jù)前人的研究成果, 東秦嶺-大別地區(qū)中生代巖漿巖的形成時代主要集中于 145～110 Ma之間[11–20]。大別造山帶中生代巖漿活動特別強烈, 是世界上超高壓變質(zhì)地體中碰撞后花崗質(zhì)巖漿活動最為強烈的地區(qū)[21], 大規(guī)模的晚中生代花崗巖類出露面積約占區(qū)域面積的 17%[22]。在 145～130 Ma形成的花崗巖體, 常具有高 Sr、Ba、La/Yb, 低 Y、Yb的地球化學(xué)性質(zhì), 被稱為“埃達(dá)克質(zhì)巖”; 在130～110 Ma形成的普通花崗巖, 具有明顯虧損 Ba、Sr、Eu的地球化學(xué)特征[11,19,23,24]; 而且, 在130～123 Ma, 基性侵入體也比較發(fā)育[25–31]。這兩個階段的花崗巖可能分別形成于碰撞和地殼加厚之后的伸展隆升階段,代表了不同的深部地殼大陸動力學(xué)過程。不同階段花崗巖類的巖石地球化學(xué)和同位素地球化學(xué)特征很可能記錄了不同的熱源和物源對花崗巖漿作用的貢獻(xiàn), 進(jìn)而能制約造山帶的演化歷程。

商城和達(dá)權(quán)店花崗巖位于秦嶺-大別造山帶東段的北大別構(gòu)造單元和北淮陽構(gòu)造單元的結(jié)合部位,兩個巖體相鄰產(chǎn)出, 且在形成時代上分別介于上述145～130 Ma和130～110 Ma兩個不同的花崗巖形成階段1),[32,33], 但迄今對它們的研究程度較低, 其成因類型、源區(qū)特征及構(gòu)造環(huán)境尚不明確, 對它們進(jìn)行系統(tǒng)的地質(zhì)-地球化學(xué)研究將有助于深入探討秦嶺-大別造山帶中生代構(gòu)造-巖漿演化過程及其動力學(xué)機(jī)制。本文擬通過系統(tǒng)的鋯石 U-Pb同位素年代學(xué)、鋯石Lu-Hf同位素以及主、微量地球化學(xué)研究對這一問題進(jìn)行探討。

1 地質(zhì)背景

圖1 河南商城和達(dá)權(quán)店花崗巖體地質(zhì)圖(據(jù)文獻(xiàn)[34]、河南省地礦局第三地質(zhì)調(diào)查隊1) 河南地礦廳區(qū)域地質(zhì)調(diào)查隊, 1∶5萬商城縣幅、達(dá)權(quán)店幅中華人民共和國區(qū)域地質(zhì)調(diào)查報告, 1992。 河南省地礦局第三地質(zhì)調(diào)查隊, 1∶25萬河南省南部地質(zhì)圖, 2007。)Fig.1 Geological map of the Shangcheng and Daquandian granites (after [34] and the Third Geological Survey Team, Henan Provincial Bureau of Geo-Exploration and Mineral Development1) 河南地礦廳區(qū)域地質(zhì)調(diào)查隊, 1∶5萬商城縣幅、達(dá)權(quán)店幅中華人民共和國區(qū)域地質(zhì)調(diào)查報告, 1992。 河南省地礦局第三地質(zhì)調(diào)查隊, 1∶25萬河南省南部地質(zhì)圖, 2007。)

商城和達(dá)權(quán)店巖體位于秦嶺-大別造山帶東延的北大別隆起帶和北淮陽褶皺帶的結(jié)合部位, 屬大別造山帶范疇(圖1)。商城巖體出露面積約131 km2[32],主要為似斑狀黑云母二長花崗巖, 侵位于下奧陶統(tǒng)龜山巖組、泥盆紀(jì)南灣巖組、石炭系楊小莊組和胡油坊組中, 被后期形成的下白堊統(tǒng)金剛臺組火山巖覆蓋。達(dá)權(quán)店巖體位于商城巖體南部, 出露面積141 km21)河南地礦廳區(qū)域地質(zhì)調(diào)查隊, 1∶5萬商城縣幅、達(dá)權(quán)店幅中華人民共和國區(qū)域地質(zhì)調(diào)查報告, 1992。, 主要為二長花崗巖, 區(qū)域上侵入下元古界大別雜巖, 中元古界滸灣巖組, 震旦系-下奧陶統(tǒng)肖家廟巖組及早白堊世樸店花崗巖。區(qū)域地層巖石普遍經(jīng)受過中-深區(qū)域變質(zhì)作用, 變質(zhì)作用達(dá)角閃巖相。區(qū)域構(gòu)造以推覆構(gòu)造和斷裂構(gòu)造為主, 主要有東西向的桐柏-商城斷裂和近南北向的商城-麻城斷裂(圖1)。區(qū)域巖漿巖以早白堊世中晚期的中酸性侵入巖為主, 商城和達(dá)權(quán)店巖體均為該區(qū)域較大的侵入巖體, 巖體中偶見少量后期侵入的基性巖墻。

2 巖石學(xué)特征

商城花崗巖體由 3期侵入體組成, 第 1期侵入體位于巖體東南部, 呈不規(guī)則橢圓形, 面積約 34 km2, 巖性為巨斑狀粗粒黑云母二長花崗巖(圖 2a),似斑狀結(jié)構(gòu), 斑晶由鉀長石組成, 含量 10%～20%,基質(zhì)為粗中粒花崗結(jié)構(gòu), 塊狀構(gòu)造; 第 2期侵入體位于巖體中部, 面積約 71 km2, 巖性為似斑狀中粒黑云母二長花崗巖, 似斑狀結(jié)構(gòu), 斑晶由鉀長石組成, 含量為5%～15%, 基質(zhì)為中?；◢徑Y(jié)構(gòu), 塊狀構(gòu)造; 第3期侵入體位于巖體外圍, 呈環(huán)狀分布, 面積約26 km2, 巖性為似斑狀細(xì)中粒黑云母花崗閃長巖,似斑狀結(jié)構(gòu), 鉀長石斑晶含量為 5%～8%, 具卡斯巴雙晶(圖2e), 分布均勻, 基質(zhì)具中細(xì)?；◢徑Y(jié)構(gòu), 塊狀構(gòu)造。3個期次侵入體的基質(zhì)均由斜長石(33%～60%)、鉀長石(10%～30%)、石英(20%～30%)和黑云母(1%～5%)組成(圖2c), 有少量角閃石。斜長石均為奧長石, 多為半自形-他形粒狀, 具有聚片雙晶及環(huán)帶結(jié)構(gòu)(圖 2d); 鉀長石多為他形; 石英為他形粒狀, 波狀消光; 黑云母為半自形片狀-鱗片狀;副礦物主要為磁鐵礦、榍石、磷灰石和鋯石。含有不同程度的富云包體和暗色微粒包體, 富云包體主要由黑云母和斜長石組成, 暗色微粒包體主要由斜長石、角閃石、輝石和黑云母組成。

達(dá)權(quán)店花崗巖體位于商城花崗巖體的南部, 分布面積達(dá)141 km2, 由4個期次侵入體組成: 第1期侵入體分布于達(dá)權(quán)店巖體的北部和東南部, 分布面積約為76 km2, 巖性為似斑狀-含斑二長花崗巖, 似斑狀結(jié)構(gòu), 斑晶為鉀長石、條紋長石, 基質(zhì)為花崗結(jié)構(gòu), 局部有交代殘留、凈邊、蠕蟲結(jié)構(gòu), 塊狀構(gòu)造;第 2期侵入體分布于巖體的南部和東南部, 分布面積約為 29 km2, 巖性為中粒二長花崗巖(圖 2b), 中?；◢徑Y(jié)構(gòu), 局部交代殘留結(jié)構(gòu), 塊狀構(gòu)造; 第3期侵入體分布于巖體的中部, 分布面積約為 23 km2,巖性為中粒-細(xì)粒二長花崗巖, 中細(xì)?；◢徑Y(jié)構(gòu), 局部有交代結(jié)構(gòu), 塊狀構(gòu)造; 第4期侵入體分布于第3期侵入體內(nèi)部, 呈巖株產(chǎn)出, 分布面積約為13 km2,巖性為細(xì)粒二長花崗巖, 細(xì)?；◢徑Y(jié)構(gòu), 塊狀構(gòu)造。4期侵入體新鮮巖石多為淺肉紅色, 主要礦物為由鉀長石、斜長石、石英和少量黑云母組成, 含少量角閃石。鉀長石多為自形-半自形, 具有卡斯巴雙晶(圖 2g); 斜長石均為奧長石, 多為自形-半自形, 具有聚片雙晶, 部分斜長石聚片雙晶具環(huán)帶狀現(xiàn)象;石英多呈他形粒狀, 蠕蟲結(jié)構(gòu), 位于鉀長石和斜長石的周圍(圖 2h); 黑云母多為半自形。副礦物主要為磁鐵礦、榍石、磷灰石及少量褐簾石和鋯石等。

3 樣品與分析方法

本次工作的樣品全部取自采石場或公路兩側(cè)的新鮮樣品, 樣品采集點散布在整個區(qū)域。選取 4個商城巖體樣品(DB-1、DB-3-1、DB-9、DB-27)和 1個達(dá)權(quán)店巖體樣品(DB-17)進(jìn)行 LA-ICP-MS 鋯石U-Pb 年齡和Lu-Hf同位素分析。挑選出的鋯石粘于樹脂中制成樣品靶, 用于鋯石 LA-ICP-MS鋯石U-Pb同位素定年及原位 Hf同位素分析。鋯石環(huán)帶及內(nèi)核信息通過鋯石的陰極射線發(fā)光(CL)圖來觀察。10個商城巖體樣品和8個達(dá)權(quán)店巖體樣品磨成200目以下粉末用于主、微量地球化學(xué)分析。

圖2 河南商城和達(dá)權(quán)店花崗巖體樣品照片和顯微照片F(xiàn)ig.2 Macro- and micro-photographs of Shangcheng and Daquandian granites

主元素和微量元素分析在中國科學(xué)院廣州地球化學(xué)研究所完成。主元素用X射線熒光光譜法(XRF)測試。分析儀器為Rigaku 100e, 分析精度優(yōu)于2%。微量元素用電感耦合等離子體質(zhì)譜儀進(jìn)行測試, 分析儀器為美國 Perkin Elmer公司產(chǎn)的 Elan6000 ICP-MS, 分析精度可達(dá) 5%。在質(zhì)譜分析過程中用Rh元素作為內(nèi)標(biāo)來進(jìn)行內(nèi)部校正, 純的元素標(biāo)樣作為外部校正, 實驗室標(biāo)樣BHVO-1和SY-4作為參考標(biāo)準(zhǔn)。分析流程見文獻(xiàn)[35]。鋯石的CL圖像拍照和LA-ICP-MS鋯石U-Pb定年均在西北大學(xué)大陸動力學(xué)國家重點實驗室完成。鋯石的CL圖像拍照采用FEI公司的XL30型SFEG電子束進(jìn)行鋯石內(nèi)部結(jié)構(gòu)顯微照相分析。LA-ICP-MS鋯石 U-Pb定年測試使用的 ICP-MS為 Agiligent 7500。鋯石原位Lu-Hf同位素測定用Nu Plasma HR(Wrexham, UK)多接受電感耦合等離子體質(zhì)譜儀完成(MC-ICP-MS)。用德國MicroLas公司生產(chǎn)的 193 nm ArF準(zhǔn)分子(excimer)激光器的剝蝕系統(tǒng), 激光剝蝕以氦氣作為剝蝕物質(zhì)的載氣, 斑束直徑為 44 μm,頻率為10 Hz, 激光能量為90 mJ, 每個分析點的氣體背影采集時間為 30 s, 信號采集時間為 40 s。LA-ICP-MS激光剝蝕為單點剝蝕采樣方式。詳細(xì)的分析步驟、儀器性能、工作參數(shù)及測試條件見文獻(xiàn)[36–37]。使用國際標(biāo)準(zhǔn)鋯石 91500[38]作為外標(biāo), 每測試4～5個點插入1次標(biāo)樣測定。同位素比值和含量計算及校正使用GLITTER 4.0 軟件完成, 年齡計算以標(biāo)準(zhǔn)鋯石91500為外標(biāo)進(jìn)行同位素比值分餾校正; 元素濃度采用NIST610作外標(biāo), Si作內(nèi)標(biāo)。普通鉛矯正流程按照文獻(xiàn)[39]方法處理。本次實驗測定過程中, 91500鋯石的176Hf/177Hf的測定結(jié)果是0.282308±34, 該值與用溶液法獲得的值(0.282306±28[40])在誤差范圍內(nèi)一致。U-Pb諧和線圖和加權(quán)平均年齡的計算及繪圖用Isoplot 3.0軟件[41]完成, 處理結(jié)果見表1。

4 分析結(jié)果

4.1 鋯石CL圖像及U-Pb 同位素年代學(xué)

樣品巖性分別為似斑狀中粒黑云母二長花崗巖DB-1(地理坐標(biāo) 31°49′20″N, 115°27′18″E)、巨斑中粗粒 二 長 花 崗 巖 DB-3-1(地 理 坐 標(biāo) 31°47′18″N,115°27′54″E)、花崗斑巖 DB-9(地理坐標(biāo) 31°45′53″N,115°25′26″E)、細(xì)粒黑云母二長花崗巖 DB-27(地理坐標(biāo) 31°45′52″N, 115°13′04″E)和中粒二長花崗巖DB-17(地理坐標(biāo) 31°40′19″N, 115°21′43″E)。鋯石 CL圖像(圖3)顯示, 商城和達(dá)權(quán)店巖體中的鋯石大部分都具有繼承鋯石內(nèi)核。鋯石顏色從無色透明到淺黃色, 鋯石呈長柱狀或短柱狀, 多數(shù)呈半自形-自形,粒徑在 50～250 μm, 長寬比約為 2∶1～3∶1, 新生鋯石邊和新生鋯石有巖漿韻律環(huán)帶, 巖漿期鋯石高的Th/U比值(0.41～2.86), 具有典型的巖漿鋯石特征。

表1中列出了所有測點的鋯石U-Pb同位素數(shù)據(jù),除繼承鋯石或鋯石中繼承核的U-Pb年齡外, 較新的巖漿期鋯石U-Pb年齡均集中于早白堊世。DB-1樣品取自商城巖體第 2期侵入體, 24顆鋯石的206Pb/238U表觀年齡為133～2587 Ma(表1), 其中9顆鋯石年齡集中于133～145 Ma之間, 加權(quán)平均年齡為(139±3) Ma, MSWD = 1.1 (圖 4a)。DB-3-1 樣品取自商城巖體第1期侵入體, 24顆鋯石的206Pb/238U表觀年齡介于1907 ～133 Ma之間(表1), 其中19顆鋯石年齡集中于133～149 Ma之間, 獲得加權(quán)平均年齡為(141±2) Ma, MSWD = 3.6 (圖 4b)。DB-9 樣品取自商城巖體第1期侵入體, 20顆鋯石的206Pb/238U表觀年齡介于1859～137 Ma之間(表1), 其中12顆鋯石年齡集中于 137～147 Ma之間, 獲得加權(quán)平均年齡為(140±3) Ma, MSWD = 4.3 (圖 4c)。DB-27 樣品取自商城巖體第3期侵入體, 23顆鋯石的206Pb/238U表觀年齡介于1657～134 Ma之間(表1), 其中7顆鋯石年齡集中于 134～143 Ma之間, 獲得加權(quán)平均年齡為(137±2) Ma, MSWD = 1.7 (圖 4d)。因此認(rèn)為, 商城巖體的形成年齡為(137±2)～(141±2) Ma, 為早白堊世侵入巖, 這個年齡與 He et al.[42]得到的年齡(138±3) Ma一致。

DB-17樣品取自達(dá)權(quán)店花崗巖體的東北部, 21顆鋯石的206Pb/238U表觀年齡介于141～114 Ma之間(表 1), 主要集中在兩個年齡范圍內(nèi), 其中 9顆鋯石的206Pb/238U表觀年齡介于131～141 Ma之間, 數(shù)據(jù)獲得加權(quán)平均年齡為(136±2) Ma, MSWD = 2.6 (圖4e); 12顆鋯石的206Pb/238U表觀年齡介于 114～124 Ma之間, 數(shù)據(jù)獲得加權(quán)平均年齡為(118±1) Ma,MSWD = 2.3 (圖4e)。鋯石年齡表明達(dá)權(quán)店巖體形成于118 Ma, 而136 Ma的鋯石可能為巖漿上升過程中捕獲的鋯石, 年齡與商城巖體的形成時間接近,是這一期巖漿熱事件的反映。

王團(tuán)華等[43]測得的商城巖體中輝綠巖墻的單顆粒鋯石(采樣地點: 31°46.45′N, 115°24.44′E)具有中-新太古代到中生代多個期次的U-Pb年齡, 巖墻中兩類鋯石中的繼承鋯石年齡(284.3～216.6 Ma)與商城巖體繼承鋯石年齡基本吻合, 原生鋯石得到的多個燕山期(137.1～117.8 Ma)年齡除與商城巖體年齡接近外, 其中最年輕的年齡紀(jì)錄117.8 Ma與達(dá)權(quán)店巖體年齡接近, 兩者可能與同一期構(gòu)造熱事件過程有關(guān)。

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圖3 商城和達(dá)權(quán)店花崗巖陰極射線發(fā)光(CL)圖Fig.3 Cathodoluminance (CL) images of zircons from Shangcheng and Daquandian granites圖中圓圈為U-Pb定年及Hf同位素測試激光點位, 直徑40 μm。

圖4f為商城巖體繼承鋯石(或繼承核)年齡分布直方圖, 圖中反映商城巖體繼承鋯石的年齡主要集中于兩個峰值, 分別為200～250 Ma和650～800 Ma,且繼承鋯石年齡普遍具有較低的Th/U比值, 這兩個年齡段分別對應(yīng)大別造山帶 200～240 Ma期間的超高壓變質(zhì)作用和變質(zhì)巖快速折返的時間[2,12,44–52]和北大別混合巖化的花崗質(zhì)-英云閃長質(zhì)片麻巖的原巖年齡[53–55]。此外, 從古元古代到古太古代繼承鋯石的大量存在, 反映商城巖體的成巖物質(zhì)主要來自于較老的地殼物質(zhì)。

4.2 主元素地球化學(xué)特征

商城和達(dá)權(quán)店巖體樣品的主量元素分析結(jié)果列于表2。

商城巖體樣品具較高的SiO2(68.04%～71.93%)、Na2O (4.13%～5.40%)、K2O (3.12%～4.65%)、CaO(1.47%～2.64%)含 量 , 和 較 低 的 TiO2(0.28%～0.50%)、Fe2O3T(1.44%～2.78)和 P2O5(0.08%～0.17%)含量。商城巖體樣品Al2O3含量為 13.95%～15.93%,鋁飽和指數(shù) ACNK值(Al2O3/(CaO+Na2O+K2O)摩爾比)為0.91～1.01, ANK-ANCK分類圖(圖5)中主要落入準(zhǔn)鋁質(zhì)范圍。商城巖體具有較低的MgO (0.42%～1.08%)含量和 Mg#值(40.4～51.4)。在 K2O-SiO2圖解(圖6)中, 商城巖體的樣品均落入高鉀鈣堿性系列。且商城巖體的三個侵入期次隨時間由老到新表現(xiàn)出SiO2呈上升趨勢, 除K2O含量與SiO2含量呈正相關(guān)關(guān)系外, 其他主元素均隨 SiO2含量升高而降低, 表明各期次侵入體具有連續(xù)的演化趨勢。

圖4 商城和達(dá)權(quán)店花崗巖體LA-ICP-MS鋯石U-Pb年齡諧和圖Fig.4 LA-ICP-MS zircon U-Pb concordant diagrams for the Shangcheng and Daquandian granites

與商城巖體對比, 達(dá)權(quán)店巖體樣品具有更高的SiO2(67.18%～76.83%)含量, 更低的 CaO (0.23%～2.27%)、TiO2(0.07%～0.56%)、Fe2O3T(0.30%～2.79%)和 P2O5(0.01%～0.19%)含量, 和相似的 Na2O(4.03%～4.58%)和 K2O (3.96%～4.66%)含量, Harker圖解(圖6)中, 兩者呈連續(xù)的演化趨勢。達(dá)權(quán)店巖體的Al2O3含量為12.92%～15.55%, 鋁飽和指數(shù)ACNK值(Al2O3/(CaO+Na2O+K2O)摩爾比)為 0.98～1.11,ANK-ACNK分類圖(圖 5)中主要落入準(zhǔn)鋁質(zhì)到弱過鋁質(zhì)范圍。達(dá)權(quán)店巖體也具有低的 MgO (0.06%～0.68%)含量和中等偏高的 Mg#(32.8～49.6)值, 引起Mg#值偏高的原因也是非常低的鐵含量。在K2O-SiO2圖解(圖6)中, 達(dá)權(quán)店巖體的樣品落入高鉀鈣堿性系列。在Harker圖解(圖7)中各期次侵入體同樣表現(xiàn)出由第1期侵入體到第3期侵入體, SiO2含量呈升高趨勢, 表現(xiàn)出連續(xù)的演化趨勢。

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圖5 商城和達(dá)權(quán)店花崗巖ANK-ACNK圖解Fig.5 ANK-ACNK diagram of the Shangcheng and Daquandian granites

圖6 商城和達(dá)權(quán)店花崗巖K2O-SiO2圖解(樣品符號同圖5)Fig.6 K2O-SiO2 diagram of the Shangcheng and Daquandian granites(symbols are the same as Fig.5)

4.3 微量元素地球化學(xué)特征

商城巖體稀土總量主要介于102～216 μg/g之間,平均 180 μg/g; 達(dá)權(quán)店巖體稀土總量變化較大, 主要集中在 80.1～260 μg/g 之間, 平均 150 μg/g。兩個巖體的球粒隕石標(biāo)準(zhǔn)化稀土元素分布模式均反映輕稀土相對富集, 重稀土虧損的特征(圖8)。商城巖體的(La/Yb)N= 24.0～77.6, 平均 52.6, 達(dá)權(quán)店巖體(La/Yb)N= 5.03～55.4, 平均26.3。商城巖體樣品Eu負(fù)異常不顯著, δEu值變化于0.79～0.92之間; 達(dá)權(quán)店巖體樣品 Eu負(fù)異常亦較小, 但較商城巖體明顯(δEu= 0.43～0.84, 平均 0.67)。

在原始地幔標(biāo)準(zhǔn)化微量元素蛛網(wǎng)圖(圖8)上, 商城巖體樣品均顯示出較高的Ba (585～2069 μg/g, 平均 1560 μg/g)、Sr (546～1005 μg/g, 平均 819 μg/g)含量。達(dá)權(quán)店巖體的Ba、Sr含量變化較大, 但總體較商城巖體低, Ba = 118～2598 μg/g, 平均 1046 μg/g, Sr= 27.7～859 μg/g, 平均 313 μg/g。兩個巖體的 Y 和Yb 的含量均較低, 商城巖體 Y = 6.05～9.15 μg/g, 平均 7.84 μg/g, Yb = 0.45～0.84 μg/g, 平均 0.58 μg/g;達(dá)權(quán)店巖體 Y = 5.00～22.9 μg/g, 平均 12.0 μg/g, Yb= 0.56～2.42 μg/g, 平均 1.23 μg/g。商城巖體的 Sr/Y比值為 72.0～122, 平均 105, 較達(dá)權(quán)店巖體更高(Sr/Y = 2.52～77.4, 平均 28.5)。商城巖體明顯虧損Nb、Ta、P 和 Ti (圖 8)。

在Sr-Yb圖(圖9)上, 商城巖體樣品點大部分投影于張旗等[58]歸納出的高Sr低Yb花崗巖區(qū)域, 達(dá)權(quán)店巖體大部分投影于低Sr低Yb花崗巖區(qū)域, 且兩個巖體不同侵位期次表現(xiàn)出隨著年齡的增長Sr含量降低、Yb含量升高的趨勢, 表明隨時間源區(qū)深度逐漸變淺。

4.4 鋯石Lu-Hf同位素

選擇商城巖體3個單元已經(jīng)做過年齡測試的樣品(DB-1、DB-9、DB-27)和達(dá)權(quán)店巖體(DB-17)進(jìn)行鋯石原位 Hf同位素分析(表 3)。由于兩個巖體中大量繼承鋯石及繼承核的年齡范圍較大, 因此其繼承鋯石的Hf同位素組成范圍也較大。

商城巖體3個樣品的巖漿期鋯石的同位素組成相似,176Hf/177Hf同位素為0.281835～0.282249, 對應(yīng)的 εHf(t)值均為負(fù)值, 集中于–30.2～ –15.6(表 3, 圖10), 兩階段模式年齡集中于 2174～3079 Ma之間。達(dá)權(quán)店巖體巖漿期鋯石中的176Hf/177Hf同位素為0.282050～0.282232, 對應(yīng)的 εHf(t)值也均為負(fù)值, 集中于–23.2～ –16.7 之間(表 3, 圖 10), 兩階段模式年齡集中于2227～2628 Ma之間。Hf同位素數(shù)據(jù)表明兩個巖體均主要來自新太古代至古元古代陸殼物質(zhì)。

圖7 商城和達(dá)權(quán)店花崗巖主元素與SiO2的相關(guān)性圖解(樣品符號同圖5)Fig.7 Co-variation of SiO2 with major elements and ratios for the Shangcheng and Daquandian granites (symbols are the same as Fig.5)

兩巖體的繼承鋯石 Hf同位素的εHf(t)值和兩階段模式年齡具有較大的變化范圍(圖 11), εHf(t)值以負(fù)值為主, 兩階段模式年齡主要集中于新太古代-古元古代。

圖8 商城和達(dá)權(quán)店巖體原始地幔標(biāo)準(zhǔn)化微量元素蛛網(wǎng)圖與球粒隕石標(biāo)準(zhǔn)化稀土元素分布模式圖Fig.8 PM-normalised incompatible element spider diagrams and chondrite-normalized REE patterns of the Shangcheng and Daquandian granites

圖9 花崗巖Sr-Yb分類圖(據(jù)文獻(xiàn)[58])Fig.9 The classification of granitoids on the basis of Sr and Yb contents (after [58])

5 討 論

5.1 成因類型

商城和達(dá)權(quán)店巖體的巖性主要為斑狀黑云母二長花崗巖、二長花崗巖, 兩巖體均屬于具有高硅、富堿特征的高鉀鈣堿性、準(zhǔn)鋁至弱過鋁質(zhì)花崗巖(ACNK = 0.91～1.11)。兩者主、微量元素地球化學(xué)特征相似, 除K2O外, 其他主元素均與SiO2含量呈負(fù)相關(guān)關(guān)系。由花崗巖主元素含量和Zr含量計算的鋯石飽和溫度大部分集中于 755～830 ℃之間(平均 774℃), 由于巖體富含繼承鋯石, 源區(qū)中鋯石是飽和的,由于其Zr含量一部分是在繼承鋯石中, 而不全在熔體中, 因此計算得到的TZr代表了巖漿溫度的上限[70]。

在Nb-Ga/Al和Y-Ga/Al圖解中, 兩個巖體主要投影于I型和S型花崗巖區(qū)域, 僅有個別樣品落于I型和S型與A型花崗巖的交界處(圖12a和12b)。在FeOT/MgO-(Zr+Nb+Ce+Y)圖解中, 兩巖體普遍落于未分異花崗巖范圍, 與 A型花崗巖和分異型花崗巖不同(圖 12c)。較低的鋯石飽和溫度(755～830 ℃(平均774 ℃))也與A型花崗巖高溫的特征[72–73]有很大不同。兩巖體均含有I型花崗巖的特征礦物角閃石,副礦物以榍石、磁鐵礦、磷灰石常見, 缺少 S型花崗巖的特征富鋁礦物, 且鋁飽和指數(shù)ACNK值普遍小于1.1, 表明商城和達(dá)權(quán)店巖體具有I型花崗巖特征。此外, 實驗研究表明, P2O5在弱過鋁質(zhì)和強過鋁質(zhì)巖漿中隨 SiO2增加變化趨勢不同, 這種性質(zhì)也被成功地用于區(qū)分I型和S型花崗巖類[74–77]。在P2O5隨SiO2變化的圖解(圖7)上, 數(shù)據(jù)點總體沿I型演化趨勢分布, 即P2O5與SiO2含量呈負(fù)相關(guān)關(guān)系。表明商城和達(dá)權(quán)店花崗巖均為高鉀鈣堿性的 I型花崗巖。

表3 商城和達(dá)權(quán)店花崗巖鋯石Hf同位素數(shù)據(jù)Table 3 Zircon Hf isotopic compositions of the Shangcheng and Daquandian granites

(續(xù)表 3)

圖10 商城和達(dá)權(quán)店花崗巖體εHf(t)-t圖解Fig.10 Plots of zircon εHf(t) values vs U-Pb age for the Shangcheng and Daquandian granites

5.2 巖石成因

根據(jù)微量元素和稀土元素分析, 商城巖體具有較典型的埃達(dá)克巖[78,79]的地球化學(xué)特征, 如 SiO2＞56%、A12O3＞ 15%、Na2O ≥ 3.3%、高 Sr (＞ 400 μg/g)、不明顯的 Eu負(fù)異常(0.79～0.92)、低 Y 和 Yb (Y＜ 15 μg/g、Yb ＜ 1.9 μg/g)、富集 LREE、虧損 HREE等, 并具有較低的 Mg#值(40.4～51.4), 與大別山廣泛出露的早白堊世低鎂埃達(dá)克巖地球化學(xué)特征相似[23]。低的HREE、Y和Yb含量, 表明源區(qū)有石榴子石殘留[80]。Sr在石榴子石、角閃石和單斜輝石中分配系數(shù)很小(分別為 0.015、0.058和 0.2), 而在斜長石中很大, 因此巖石的Sr正異常和高Sr/Y比值表明巖漿源區(qū)的斜長石在巖漿形成過程中由于高壓條件發(fā)生分解作用進(jìn)入熔體[79]。樣品對原始地幔的蛛網(wǎng)圖上可以看出商城巖體明顯虧損Nb、Ta和Ti, 這可能與金紅石的分離結(jié)晶或源區(qū)殘留有關(guān)[81–82]。實驗研究表明, 當(dāng)玄武巖熔融形成含金紅石的榴輝巖時, 熔體強烈虧損 Nb和 Ta[83–84], 因此殘留相中金紅石的產(chǎn)出會導(dǎo)致熔融的花崗巖貧 Nb、Ta、Ti等元素[85–87]。在 Sr/Y-Y和(La/Yb)N-YbN圖解(圖 13)中, 商城巖體的樣品點大部分投影于太古宙TTG與埃達(dá)克巖范圍內(nèi), 且在Sr/Y-Y圖上全部落于加厚下地殼熔融形成的埃達(dá)克巖范圍內(nèi)。從圖上可以看出, 商城巖體主要落于含 10%石榴子石的角閃巖相和榴輝巖相的熔融曲線之間, 表明巖石主要是由榴輝巖或石榴石角閃巖相的基性巖石發(fā)生低程度部分熔融作用形成。

結(jié)合熔融曲線和商城巖體的地球化學(xué)特征, 推測殘留相主要以石榴子石、角閃石、金紅石為主。石榴子石穩(wěn)定出現(xiàn)的壓力至少大于0.8～1.0 GPa, 通常大于1.5 GPa(以金紅石的出現(xiàn)為標(biāo)志)[80], 因此商城巖體巖漿部分熔融壓力大于1.5 GPa, 相應(yīng)的深度至少大于50 km。

圖11 商城和達(dá)權(quán)店巖體鋯石Hf同位素εHf(t)和兩階段模式年齡直方圖Fig.11 Histograms of zircon εHf(t) and tDM2 for the Shangcheng and Daquandian granites

圖12 商城和達(dá)權(quán)店巖體Nb-Ga/Al、Y-Ga/Al和FeOT/MgO-(Zr+Nb+Ce+Y)圖解(據(jù)文獻(xiàn)[71], 樣品符號同圖5)Fig.12 Nb vs. Ga/Al, Y vs. Ga/Al and FeOT/MgO vs. Zr+Nb+Ce+Y diagrams of the Shangcheng and Daquandian granites (after [71]; symbols are the same as Fig.5)

圖13 商城和達(dá)權(quán)店巖體的Sr/Y-Y和(La/Yb)N-YbN圖解(據(jù)文獻(xiàn)[88]; 樣品符號同圖5)Fig.13 Plots of Sr/Y vs Y and (La/Yb)N vs YbN for the Shangcheng and Daquandian granites (after [88]; symbols are the same as Fig.5)

近年來的研究表明, 玄武質(zhì)巖漿底侵提供熱源促使加厚的下地殼(＞50 km)基性巖石的部分熔融[79–94], 以及拆沉作用引起的下地殼玄武質(zhì)巖石的部分熔融[95–99]可能是中國東部增厚下地殼減薄的兩個重要機(jī)制。在主要氧化物和微量元素與SiO2圖解(圖 14)中, 商城巖體的成分點均落入增厚下地殼熔融形成的埃達(dá)克質(zhì)巖區(qū)域內(nèi), 與拆沉下地殼熔融形成的埃達(dá)克質(zhì)巖差別較大。因此, 商城巖體主要是幔源巖漿底侵促使下地殼部分熔融形成的。根據(jù)Barbarin[101]的花崗巖類分類方案, 商城巖體屬于富鉀的鈣堿性斑狀二長花崗巖類(K-rich and K-feldspar porphyritic calc-alkaline granitoids; KKG), 這類花崗巖能夠很好地指示地球動力學(xué)背景, KGG形成于造山帶造山作用結(jié)束后大陸板塊從擠壓向伸展轉(zhuǎn)換的構(gòu)造環(huán)境, 標(biāo)志著大別造山帶垮塌的開始, 即厚的地殼伸展減薄的開始。

圖14 商城和達(dá)權(quán)店巖體主元素與SiO2圖解(據(jù)文獻(xiàn)[23,88,100]; 樣品符號同圖5)Fig.14 Major elements vs.SiO2 plots of the Shangcheng and Daquandian granites (after [23,88,100]; symbols are the same as Fig.5)

商城和達(dá)權(quán)店巖體都具有多階段多期次侵入的特點, 且兩個巖體各期次侵入體的巖石學(xué)和地球化學(xué)特征均有所差別, 即晚期侵入體相比早期侵入體含有更高的SiO2含量、ACNK值, 低的稀土元素總量、Sr含量和Sr/Y比值。這些地球化學(xué)特征的演變,反了從早期侵入體到晚期侵入體, 巖漿演化程度更高, 形成壓力逐漸降低, 形成深度相對變淺。其中,達(dá)權(quán)店巖體是燕山晚期巖漿活動的產(chǎn)物, 比商城巖體具有更高的SiO2含量, 低的Ba、Sr含量, 同時虧損 Nb、Ta、Sr、P、Ti和 Eu等元素(圖 8), 指示其母巖漿經(jīng)歷了分離結(jié)晶演化[76]。在Ba-Sr、Rb-Sr對數(shù)圖解(圖 15)上, 達(dá)權(quán)店巖體隨著 Ba含量的升高,Sr沿鉀長石結(jié)晶分異演化線快速的從27.7 μg/g增加到 859 μg/g, 這種變化趨勢也進(jìn)一步證明了巖體在形成過程中主要經(jīng)歷了鉀長石和斜長石的分異, 而商城巖體沒有表現(xiàn)出明顯的結(jié)晶分異演化趨勢。雖然達(dá)權(quán)店巖體中Y、Yb含量較低, 相當(dāng)于埃達(dá)克質(zhì)巖的 Y、Yb含量, 且虧損重稀土, 表明源區(qū)可能有石榴子石存在。但巖體貧Sr、低Al, 有比較明顯的Eu負(fù)異常, 說明殘留相中有斜長石存在。在 Sr-Yb分類圖解(圖9)上, 達(dá)權(quán)店巖體主要分布于張旗等[58]歸納的低Sr、低Y花崗巖范圍, 是在中等壓力下形成的, 較商城巖體巖漿來源深度淺, 推測其殘留相為斜長石、石榴子石、輝石, 表明此時地殼厚度較商城巖體形成時明顯減薄。Sr/Y-Y圖解(圖 13)中落在埃達(dá)克巖和正常島弧安山巖、英安巖、流紋巖的過渡區(qū)域, 說明了其形成于厚的地殼向正常地殼厚度過渡或轉(zhuǎn)變的環(huán)境(與斜長石相平衡的源區(qū)環(huán)境)。從商城巖體到達(dá)權(quán)店巖體從早到晚形成的源區(qū)環(huán)境由石榴子石相平衡向斜長石相平衡的轉(zhuǎn)變, 即地殼由厚變薄。

5.3 物質(zhì)源區(qū)

鋯石Hf同位素特征顯示, 商城和達(dá)權(quán)店巖體巖漿期鋯石的 εHf(t)值分別集中于–30.2～ –15.6 和–23.2～ –16.7 之間, tDM2值分別為 2173～3079 Ma和2227～2628 Ma, 表明兩個巖體源于古老的地殼巖石的部分熔融。Zhang et al.[103]根據(jù)Sr-Nd-Pb同位素組成對比認(rèn)為大別山白堊紀(jì)不同構(gòu)造單元花崗巖類的同位素組成相似, 并且與巖石類型關(guān)系不大, 它們應(yīng)來源于基本相似的深部陸殼物質(zhì)源區(qū)。且不同構(gòu)造單元早白堊紀(jì)花崗巖類的εHf值 (t = 128～132 Ma)= –15.1～ –30.5, tDM2= 2133～3100 Ma[19,65]與商城和達(dá)權(quán)店巖體的Hf同位素組成也基本一致, 表明商城和達(dá)權(quán)店巖體應(yīng)均來源于與大別山其他地區(qū)早白堊世花崗巖體基本相似的深部陸殼物質(zhì)源區(qū)(圖16)。

圖15 商城和達(dá)權(quán)店巖體造巖礦物結(jié)晶分異判別圖解(據(jù)文獻(xiàn)[102]; 樣品符號同圖5)Fig.15 Ba vs Sr and Rb vs Sr plots for the Shangcheng and Daquandian granites (after [102]; symbols are the same as Fig.5)

圖16 商城和達(dá)權(quán)店巖體與北大別白堊紀(jì)花崗巖Hf同位素組成對比(北大別早白堊紀(jì)花崗巖Hf同位素組成范圍根據(jù)文獻(xiàn)[19,63,65]Fig.16 Hf isotopic composition of the Shangcheng, Daquandian and North Dabie Cretaceous granites (Hf isotopic data of the North Dabie Cretaceous granites are from [19,59,61]

商城巖體和達(dá)權(quán)店巖體巖漿期鋯石的 εHf(t)值具有很大的變化范圍(圖 10), 表明源區(qū)組成并不均一, 原始巖漿不是單一組分。變化很大的繼承鋯石年齡也表明混入不同年齡的地殼物質(zhì)在花崗巖源區(qū)中。商城巖體存在大量繼承鋯石, 除了一個 3.1Ga和兩個約2.5 Ga的年齡之外, 其他繼承鋯石年齡主要集中于三疊紀(jì)、新元古代和約1.7～2.0 Ga的古元古代中晚期, 與大別-蘇魯造山帶各構(gòu)造單元廣泛出露的早白堊世巖漿巖的繼承鋯石年齡情況基本一致[17,19,20,104,105]。其中新元古代(約750 Ma)和三疊紀(jì)(約 230 Ma)的繼承鋯石 U-Pb年齡與大別蘇魯造山帶超高壓變質(zhì)巖的原巖和變質(zhì)年齡一致[53,106,107],古元古代(1.7～2.0 Ga)和太古宙(2.5～3.1 Ga)U-Pb 年齡與崆嶺雜巖中TTG片麻巖和混合巖的原巖侵位年齡一致[17,66–68,108,109], 這些年齡特征暗示商城和達(dá)權(quán)店巖體與大別超高壓變質(zhì)巖及崆嶺雜巖可能具有親源性。

前人研究表明大別-蘇魯造山帶早白堊世后碰撞花崗巖與北大別超高壓片麻巖具有相似的Sr、Nd同位素組成[103–104]。Ma et al.[110]也認(rèn)為早白堊世花崗巖和花崗斑巖為大別雜巖深熔的產(chǎn)物。Bryant et al.[17]認(rèn)為花崗巖不僅由北大別片麻巖部分熔融形成,源巖中還需要更加古老的地殼物質(zhì), 如太古宙崆嶺雜巖。大別雜巖是大別造山帶穹窿構(gòu)造的核部組成,其中北大別雜巖是大別核部雜巖單元的主要組成部分, 由以灰色片麻巖為主的長英質(zhì)片麻巖、以斜長角閃巖為主的鐵鎂質(zhì)巖和基性麻粒巖組成, 主體變質(zhì)屬于角閃巖相, 且近年研究提出作為核雜巖主體的長英質(zhì)片麻巖中存在原巖為巖漿成因的 TTG巖系[111–113]。長英質(zhì)片麻巖的原巖年齡為 700～800 Ma[114,53–55], 混合巖化年齡為 120～140 Ma[11–12]; 殘存的麻粒巖原巖年齡為 2600～2700 Ma和 3400 Ma(繼承核), 早期麻粒巖相變質(zhì)年齡為 2000～2200 Ma[115–116], 主要的鋯石 Hf模式年齡為古元古代(1.9 Ga)[69]。前人普遍認(rèn)為北大別雜巖的原巖屬性是被新元古代 Rodinia超大陸裂解改造的揚子克拉通北緣大陸俯沖陸殼基底, 并夾雜少量(古)太古宙古老殘片, 曾經(jīng)受過榴輝巖相變質(zhì)作用[11,12,53,55,117]。目前已知的揚子克拉通北緣最古老的基底是崆嶺群高級變質(zhì)地體。崆嶺地體主要的巖石組合為 TTG片麻巖,變沉積巖和局部保存有基性麻粒巖的斜長角閃巖。對崆嶺變質(zhì)巖進(jìn)行的多種方法的定年工作認(rèn)為其原巖形成時代為 2.85～3.2 Ga[66,68,109,118–121]。此外, 高山等[122]還給出了1992 Ma的變質(zhì)年齡和3.3 Ga的碎屑鋯石。這些鋯石具有負(fù)的εHf(t)值和古老的兩階段模式年齡(3.6 Ga)(圖10)。路鳳香等[94]利用Pb同位素對比和巖漿熔融模擬計算認(rèn)為大別下地殼中存在類似崆嶺群的巖層, Ma et al.[21]通過虧損地幔與崆嶺群TTG片麻巖兩端元混合計算模擬, 也得出崆嶺群可能廣泛分布于大別造山帶中生代地殼深部的認(rèn)識。

商城和達(dá)權(quán)店巖體的同位素特征顯示巖漿期鋯石 Hf同位素的兩階段模式年齡分別為 2174～3079 Ma和2227～ 2628 Ma, 明顯老于北大別片麻巖古元古代的鋯石 Hf模式年齡(1.9 Ga)[69], 因此, 花崗巖不應(yīng)該只形成于北大別片麻巖的部分熔融[123], 而需要包括更多古老太古宙地殼物質(zhì)。圖10中, 所有的花崗巖的鋯石 εHf(t)值都在北大別片麻巖和崆嶺群TTG片麻巖的地殼演化線之間, 這表明花崗巖主要來自于北大別片麻巖, 并可能含有更古老的崆嶺群物質(zhì)。

5.4 構(gòu)造環(huán)境

大量的地質(zhì)年代學(xué)研究表明, 大別造山帶廣泛出露的高壓-超高壓榴輝巖相變質(zhì)作用主要出現(xiàn)在240～220 Ma[45–52,124]。這些年齡被看作為揚子克拉通陸殼向北俯沖于華北克拉通之下[2]以及超高壓變質(zhì)作用之后巖石剝露和冷凝的時間[125]。在板塊聚合的過程中, 大陸板塊的深俯沖/碰撞普遍導(dǎo)致大陸造山帶下加厚地殼的形成[126], 從擠壓向伸展的轉(zhuǎn)換則更晚。Liu et al.[127]通過對大別山地區(qū)中生代的盆地分析認(rèn)為伸展作用可能始于晚侏羅世-早白堊世左右, 馬昌前等[128]通過巖漿作用的研究進(jìn)一步指出碰撞后強烈增厚的地殼開始趨于伸展減薄, 伸展作用可能始于早白堊世。

大別山地區(qū)有若干同商城巖體地球化學(xué)特征相似, 也具有高Sr/Y比值和虧損重稀土元素特征的埃達(dá)克質(zhì)巖侵位, 其鋯石 U-Pb年齡介于 140～130 Ma之間[11,12,15,20,28,30,128,129], 這些花崗巖體被認(rèn)為是增厚的玄武質(zhì)下地殼部分熔融的產(chǎn)物, 但此時的構(gòu)造環(huán)境已經(jīng)不是擠壓體制。商城巖體侵位年齡為137～141 Ma, 屬于富鉀的鈣堿性二長花崗巖類, 形成于造山帶造山作用結(jié)束后大陸板塊從擠壓向伸展轉(zhuǎn)換的構(gòu)造環(huán)境, 標(biāo)志著大別造山帶垮塌的開始,即厚的地殼伸展減薄的開始。巖石圈減薄可以使地殼巖石因減壓而熔融[130–131]。然而, 沒有深部地幔物質(zhì)或熱源的供給, 單純增厚地殼的減壓熔融只能形成小規(guī)模的巖體, 缺少足夠的熱量不可能形成大規(guī)模的巖漿活動[131–132], 應(yīng)有外來熱源的加入使巖漿源區(qū)大規(guī)模熔融[70]。通常認(rèn)為幔源巖漿活動(特別是基性巖漿的底侵和侵入)提供的熱是導(dǎo)致地殼物質(zhì)重熔形成花崗巖的重要因素, 地幔與地殼之間的熱傳遞被認(rèn)為是地殼熔融的主要方式, 而最有效的方式是通過幔源巖漿底侵作用將地幔熱傳遞到下地殼引發(fā)熔融形成中酸性巖漿[133], 同時伴隨基性巖漿作用。

前人對大別地區(qū)零星分布的早白堊世輝石巖-輝長巖侵入體、基性巖墻/脈和基性火山巖的研究得到這一期基性巖漿活動主要集中于 120～130 Ma范

圍[27,30,93,109,134–138]。陳道公等[139]對大別山北部道士沖輝石巖進(jìn)行了鋯石微區(qū)離子探針 U-Pb定年研究,得到年齡變化于159～134 Ma之間, 且大量測試點的年齡數(shù)據(jù)主要集中于138～148 Ma范圍, 加權(quán)平均年齡是(144.5±6.2) Ma, 與商城巖體形成年齡相近。巖石地球化學(xué)研究表明道士沖輝石巖母巖漿為地?；匀垠w, 并經(jīng)歷了與基性下地殼物質(zhì)的相互作用[140], 表明大別地區(qū)早白堊世巖漿活動最早可以追溯到約 145 Ma。這與早白堊世花崗巖同位素年齡集中于111～143 Ma范圍, 但峰期在 125～130 Ma相對應(yīng)[11–20,104], 表明深部基性熔體是促使下地殼發(fā)生部分熔融形成早白堊世花崗巖的主要熱源。

馬昌前等[128]根據(jù)大別山地區(qū)高 Sr/Y比值花崗巖年齡為(135.7±2.7) Ma, 而鐵鎂質(zhì)巖體和性質(zhì)類似達(dá)權(quán)店花崗巖體的具有 Sr-Eu負(fù)異常的花崗巖的同位素年齡為105～130 Ma, 認(rèn)為大別山造山帶的減薄作用發(fā)生在約135 Ma。Xie et al.[18]認(rèn)為北大別花崗巖體鋯石 U-Pb年齡(128±2) Ma代表最小拉張起始時間。吳元保等[141]根據(jù)北大別混合巖化作用的年齡結(jié)果, 認(rèn)為大別地區(qū)構(gòu)造體制轉(zhuǎn)換的時間為(137±4)Ma。由于伸展作用引起的地殼部分熔融和花崗巖侵位一般會稍晚于部分熔融作用發(fā)生的時間, 所以一般把加厚地殼開始發(fā)生熔融作用的時間看作為構(gòu)造體制轉(zhuǎn)換發(fā)生的時間的最小值[142–144]。因此, 根據(jù)商城巖體的年齡結(jié)果, 我們認(rèn)為北淮陽地區(qū)構(gòu)造體制轉(zhuǎn)換的最小時間為(137±2) Ma。

續(xù)海金等[65]根據(jù)早白堊紀(jì)兩期花崗巖類的形成時間間隔認(rèn)為大別造山帶在很短的幾個 Ma內(nèi)(約4～6 Ma)快速伸展垮塌, 所得結(jié)論主要來自北大別地區(qū)的花崗巖體。本文研究的達(dá)權(quán)店巖體的形成年齡為(118±1) Ma, 為過鋁質(zhì)的高鉀鈣堿性花崗巖,部分熔融深度明顯比商城巖體淺, 形成于厚的地殼向正常地殼厚度過渡或轉(zhuǎn)變的環(huán)境, 但地殼厚度可能仍大于正常地殼厚度, 表明研究區(qū)在(118±1) Ma時, 加厚地殼雖已經(jīng)減薄, 但還未徹底伸展垮塌。由此可見, 北淮陽地區(qū)的加厚地殼的構(gòu)造體制演化過程在時間上與北大別地區(qū)有所不同。

此外, 商城地區(qū)花崗巖體中常見輝綠巖墻, 這些基性巖墻的SHRIMP鋯石U-Pb年齡范圍很寬, 但主要為燕山期年齡(137.1～117.8 Ma)[42]。這類巖墻/巖脈在北淮陽地區(qū)十分常見, 形成時代集中于約120 Ma[145], 與巖石圈伸展環(huán)境下富集地幔的部分熔融有關(guān)[28,28,146]?；詭r墻的侵位代表了強烈的伸展作用已經(jīng)開始, 進(jìn)一步證明了深部地幔物質(zhì)是下地殼部分熔融的熱源[131–132]。

從商城巖體到達(dá)權(quán)店巖體, 巖體形成壓力逐漸降低, 反映了花崗巖漿活動的源區(qū)或巖漿房逐漸向淺部遷移。而其后基性巖墻、巖脈的侵入, 表明巖漿源區(qū)由殼源向幔源演化。商城和達(dá)權(quán)店巖體的形成反映了地殼從擠壓加厚向伸展減薄轉(zhuǎn)換的過程。

6 結(jié) 論

(1) 商城和達(dá)權(quán)店花崗巖體位于西大別隆起帶北部, 主要巖性為二長花崗巖, 鋯石的U-Pb諧和年齡分別為(137±2)～(141±2) Ma和(118±1) Ma。兩者地球化學(xué)特征相似, 均為準(zhǔn)鋁質(zhì)-弱過鋁質(zhì)高鉀鈣堿性I型花崗巖。

(2) 兩巖體均具有輕稀土富集、重稀土虧損的稀土元素分布模式及Nb、Ta負(fù)異常特征。商城巖體Eu負(fù)異常不顯著, Sr、Ba含量較高, Y和Yb含量較低, Sr/Y比值高, 是加厚下地殼熔融的產(chǎn)物, 巖漿來源深度大于50 km。達(dá)權(quán)店巖體Eu負(fù)異常明顯, Sr、Ba、Y和Yb含量及Sr/Y比值均較低, 巖漿來源深度較淺, 形成時期地殼厚度較商城巖體形成時期明顯減薄。從早白堊世早期 的 商城巖體到早白堊晚期達(dá)權(quán)店巖體的侵入, 反映了花崗巖漿活動的源區(qū)或巖漿房逐漸向淺部遷移, 及地殼從擠壓加厚向伸展減薄轉(zhuǎn)換的過程

(3) 通過繼承鋯石及Hf同位素組成的對比認(rèn)為,兩個巖體的源巖均主要來自北大別片麻巖, 并可能含有更古老的崆嶺群物質(zhì)。

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