姚勇 于胜尧 纪文涛 李妍 李卓凡
摘 要:柴北缘乌兰地区出露有大量早古生代中低压泥质麻粒岩。据岩石学和矿物化学研究,将乌兰地区早古生代泥质麻粒岩变质作用划分为两期。其中峰期变质阶段矿物组合为Grt1+Bt1+Sil+Kfs+Pl+Qtz+melt,退变质阶段矿物组合为Bt2+Ms+Grt2+H2O±Pl±Qtz。通过对泥质麻粒岩样品进行锆石LA-ICP-MS U-Pb定年,获得泥质麻粒岩峰期变质时代为(482±3) Ma。为厘清乌兰地区早古生代泥质麻粒岩变质演化历史,揭示麻粒岩变质机制,通过相平衡模拟技术和黑云母Ti温度计,获得乌兰地区早古生代泥质麻粒岩峰期和退变质温压条件分别为:710 ℃~730 ℃/5~6.2 kbar和560 ℃~650 ℃/5~6.2 kbar。由于岩浆的冷却使其退变质阶段经历等压降温变质作用。
关键词:柴北缘;乌兰地区;早古生代;麻粒岩相
研究表明,柴北缘经历了洋壳俯冲到陆壳俯冲及俯冲板片折返的整个过程[1-4]。目前对南祁连洋形成时间、演化过程等仍存在争议[1-8]。关于柴北缘变质岩的研究主要集中在高压-超高压变质岩,特别是榴辉岩,前人已做了较详细工作。如榴辉岩变质年龄为420~460 Ma,陆壳榴辉岩原岩年龄为820~850 Ma,洋壳榴辉岩原岩年龄为500~540 Ma[9-14]。榴辉岩、泥质变质岩和石榴橄榄岩中发现柯石英和金刚石等超高压变质矿物,揭示柴北缘陆壳俯冲深度超过200 km[1,7,15-17]。本文以烏兰地区早古生代泥质麻粒岩为研究对象,通过野外观察、岩相学特征研究,结合锆石U-Pb定年及相平衡模拟技术,确定乌兰地区早古生代泥质麻粒岩变质时代及温压条件,厘清变质演化历史,并探讨了乌兰地区麻粒岩形成机制及动力学背景。
1 地质背景
柴北缘位于青藏高原北部,以乌兰-鱼卡断层为界被分为两个构造单元,南部为超高压变质带,北部为欧龙布鲁克微陆块。柴北缘超高压变质带中出露有大量超高压变质岩石,如榴辉岩、高压麻粒岩和泥质片麻岩等,变质时代集中在420~460 Ma[9-14]。自西向东分为4个超高压单元:鱼卡-榴辉岩片麻岩单元、绿梁山石榴橄榄岩-高压麻粒岩单元、锡铁山榴辉岩-片麻岩单元和都兰榴辉岩-片麻岩单元[9,18]。欧龙布鲁克微陆块变质基底主要由3部分组成:古元古代德令哈杂岩、古元古代达肯达坂群及中元古代万洞沟组[9,19-21]。
乌兰地区位于欧龙布鲁克微陆块东段,岩石组合主要为花岗片麻岩、副片麻岩、花岗岩,基性侵入岩、二辉麻粒岩、角闪岩等以透镜体形式出露于副片麻岩中(图1)[21-22]。早古生代乌兰地区经弧岩浆作用和麻粒岩相变质作用,岩浆岩主要包括辉长岩和花岗岩。康珍等认为肯得隆沟基性岩浆岩是由古生代俯冲熔-流体与富集岩石圈地幔发生交代作用形成的[23]。孙娇鹏通过对长英质片麻岩和透辉二长变粒岩等研究[24],认为早古生代宗务隆构造带起始时间不晚于497 Ma。Wang et al.利用传统温压计得到乌兰地区早古生代麻粒岩峰期温压为718 ℃ ~729 ℃和4.6~5.3 kbar[25],P-T轨迹为逆时针。Li et al.通过相平衡模拟方法得出研究区麻粒岩峰期变质条件为800 ℃~900 ℃和5.5~7 kbar[26],认为乌兰地区早古生代麻粒岩经顺时针P-T轨迹。
2 样品描述
2.1 岩相学特征
泥质麻粒岩WL18-4-6.1,风化面呈黑色,新鲜面呈灰白色(图2-a,b),片麻理发育,产状190°∠42°,主要由石榴子石(10%~15%)、黑云母(15%~20%)、矽线石(5%~10%)、白云母(3%~5%)、斜长石、钾长石、石英等矿物组成,长石和石英总含量超过60%,副矿物为锆石、钛铁矿和独居石,矿物简写参见Kretz(1983)[27]。黑云母多呈黄褐色,有两种形态,一种呈簇状或片状,颗粒较大,边缘处常发生破碎形成破碎条带,常有小颗粒石榴子石、矽线石和钛铁矿发育在大颗粒的黑云母聚集区域(图2-c,d),暗示此类黑云母可能发生了脱水熔融反应[28-29]:
Bt+Qtz±Pl→Grt+Sil+Kfs+Ilm+melt;
另一种黑云母呈针柱状,颗粒较小,常出现在大颗粒石榴子石边缘或裂隙里(图2-e)。矽线石也有两种形态,一种为大颗粒斑晶,内部包含黑云母、钛铁矿等包裹体,边部还存在白云母,可能发生了如下反应[30]:Sil+Kfs+H2O→Ms+Pl+Qtz;
另一种矽线石呈针柱状,常出现在其它矿物周围,可能代表其形成较晚(图2-d)。石榴子石呈半自形-自形斑晶,裂隙较发育,裂隙处通常被小颗粒黑云母填充,边缘部位常发育长石、石英、小颗粒黑云母和针柱状矽线石(图3-e,f),指示石榴子石可能发生了如下反应[31]:Grt±Kfs±melt→Bt+Sil+Qtz±Pl。
长石呈他形-半自形,在正交偏光镜下,斜长石具明显聚片双晶,钾长石具特征性卡式双晶,石英多呈他形与长石交错出现。
2.2 矿物化学
在岩相学基础上,本文对泥质麻粒岩中代表性矿物进行化学成分分析,结果见表1。
2.2.1 石榴子石
石榴子石化学成分以高FeO为主要特征,MgO、CaO和MnO相对较低。石榴子石4种端元组分中,铁铝榴石(Alm)含量最高,镁铝榴石(Prp)、钙铝榴石(Grs)、锰铝榴石(Sps)含量相对较低。(Alm+Sps)-Grs-Prp图解中,泥质麻粒岩石榴子石都落入铁铝榴石区域(图3-a)。另石榴子石核部到边部化学成分发生了明显变化,镁铝榴石含量从核部到边部逐渐降低,铁铝榴石含量从核部到边部逐渐升高,钙铝榴石含量基本保持不变(图3-b),这些端元组分变化表明岩石经等压降温变质作用。
2.2.2 黑云母
黑云母有两种形态:簇状或片状的大颗粒黑云母与针柱状小颗粒黑云母,两者矿物化学成分存在显著差异。大颗粒黑云母TiO2含量较高,一般为2.23%~3.11%,反映较高的变质温度;小颗粒黑云母TiO2含量较低,一般小于2%,反映相对低的变质温度。另两者MgO、FeO略有差异,大颗粒黑云母MgO相对略低,FeO相对略高。在黑云母分类图解中,所有测试点都落在铁黑云母区域(图3-c)。
2.2.3 长石
斜长石Na2O含量9.80%~10.77%,CaO含量2.14%~3.19%,K2O含量0.16%~0.36%。钾长石Na2O含量0.92%~2.17%,CaO含量0.01%~0.07%,K2O含量13.74%~15.45%。An-Ab-Or长石分类图解中,所有斜长石测试点都落在高更长石中,钾长石都属透长石(图3-d)。
在岩相学及矿物化学研究基础上,本文将泥质麻粒岩分为两个变质阶段。峰期矿物组合为Grt1+Bt1+Sil+Kfs+Pl+Qtz+melt,退变质矿物组合为Bt2+ Ms+Grt2+Sil+H2O±Pl±Qtz。
3 峰期变质作用P-T条件
本文选取乌兰地区泥质麻粒岩进行P-T条件相平衡模拟,相平衡模拟技术采用Perple_X6.7.9计算程序[32],热力学数据库选择hp62ver.dat[33]。涉及的矿物及熔体活度模型包括:melt(G)、Gt(W)、Mica(CHA1)、Bio(HP)、feldspar、Ilm(WPH),选择的化学体系为Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O-TiO2(NCKFSMASHT) 。据详细薄片观察并结合相应电子探针数据,我们得到相平衡模拟的有效全岩组分:SiO2=60.34%,Al2O3=13.52%,CaO=0.55%,MgO=3.00%, FeO=11.69%,K2O=1.70%,Na2O=1.63%,TiO2=4.19%,H2O=2.70%。样品中的H2O据薄片中矿物体积百分含量估算。
据相平衡模拟结果(图4),固相线出现于710 ℃~800 ℃;白云母稳定存在温度小于710 ℃~800 ℃,且压力大于5 kbar区域;黑云母在温度大于760 ℃后完全消失;斜长石在温度大于840 ℃后完全消失;金红石稳定出现于8 kbar之上;钛铁矿一般出现于9 kbar之下;石榴子石、钾长石和石英稳定在绝大多数温压范围内。乌兰泥质麻粒岩峰期矿物组合为Grt+Bt+Sil+Kfs+Pl+Qtz+melt,结合峰期石榴子石矿物成分等值线(XCa=Ca/(Ca+Mg+Fe)=0.013~0.016, XMg=Mg/(Ca+Mg+Fe)=0.12~0.15),得出乌兰地区泥质麻粒岩峰期P-T条件为710 ℃~730 ℃和5~6.2 kbar。
另利用黑云母Ti温度计得出乌兰地区泥质麻粒岩峰期变质温度为660 ℃~720 ℃[34],退变质温度为560 ℃~650 ℃。因此,我们推测乌兰地区泥质麻粒岩峰期变质P-T条件为710 ℃~730 ℃和5~6.2 kbar,退变质P-T条件为:560 ℃~650 ℃和5~6.2 kbar。
4 锆石U-Pb定年结果
本文对乌兰地区泥质麻粒岩样品WL18-4-6.1进行系统定年研究,得到33个有效测试点,锆石U-Pb测试结果见表2。锆石CL图像显示,样品WL18-4-6.1锆石以浑圆状、椭圆状或不规则状为主,斑杂状结构或补丁状结构,内部多为弱分带或面状分带,没有明显核边结构和岩浆振荡环带(图5)。锆石颗粒较大,一般100~150 μm,长宽比为1:1~2:1。锆石颗粒Th含量78×10-6~328×10-6,U含量465×10-6~5221×10-6, 相对应的Th/U为0.03~0.33(绝大多数小于0.25)。结合锆石形态特征,我们认为样品WL18-4-6.1锆石为典型变质锆石。33个测试点给出的206Pb/238U年龄在(467±5) Ma和(499±7) Ma,相应的加权平均年龄为(482±3) Ma,MSWD=0.23(图6),即乌兰地区泥质麻粒岩变质年龄为(482±3) Ma。
5 讨论
5.1 乌兰地区麻粒岩变质时代
通过锆石LA-ICP-MS U-Pb定年和SHRIMP U-Pb定年等方法,可准确测出变质岩变质时代。但变质过程中,锆石生长可发生在变质作用的任何阶段,包括进变质阶段、峰期变质阶段和退变质阶段[35-37]。由于后期退变质作用或岩浆作用影响,会对变质锆石生长阶段的判断产生干扰,准确的划分变质锆石形成时的变质阶段已成为变质岩年代学中的重点与难点。本文将乌兰地区泥质麻粒岩划分出两个变质期次:峰期变质阶段和退变质阶段,通过锆石LA-ICP-MS U-Pb定年,获得乌兰地区泥质麻粒岩的变质时代为(482±3) Ma,与前人获得的乌兰地区麻粒岩的峰期变质时代(469~475 Ma)基本一致[25,26]。另我们对研究区与泥质麻粒岩伴生的基性麻粒岩进行研究,发现基性麻粒岩的麻粒岩相与角闪岩相的过渡时间为~475 Ma(待发表资料),因此,我们推测(482±3) Ma为乌兰地区泥质麻粒岩的麻粒岩相(即峰期变质作用)的变质时代。
5.2 变质机制的探讨
前人研究表明,柴北缘地区记录了从洋壳俯冲到陆壳俯冲及随后俯冲板片折返的整个过程,并形成一条典型超高压变质带,与乌兰地区同期的高温低压变质带构成双变质带[25,26,38]。但关于乌兰地区高温低压变质带中出露的麻粒岩的变质演化过程存在不同认识,制约了对高温低压变质带的进一步理解及对区域构造演化的认识。Wang et al.认为乌兰地区泥质麻粒岩经逆时针P-T轨迹[25],Li et al.认为乌兰地区麻粒岩经顺时针P-T轨迹[26]。但顺时针的P-T轨迹通常包括升温阶段,并在随后发生近等温降压变质作用,与本文样品WL18-4-6.1所经历的等压降温变质作用过程明显不同。因此,本文更倾向于乌兰地区泥质麻粒岩经逆时针的P-T轨迹的变质演化过程。最近,在乌兰南部地区赛坝沟识别出一套自南向北分别呈洋壳、海山和海沟环境的洋壳性质的岩石组合[39],從构造环境的空间地理位置上看,乌兰地区处于陆弧环境;区域上还存在大量与乌兰地区泥质麻粒岩变质作用时间同期的弧岩浆岩。因此,我们推测乌兰地区早古生代洋壳俯冲时期处于陆弧位置。
綜合前人研究成果,认为早古生代南祁连洋向北俯冲到欧龙布鲁克微陆块之下,大量海水进入俯冲通道中,与地幔楔中的物质交代发生熔融,产生大量弧岩浆。随后,弧岩浆发生上涌,为泥质麻粒岩的原岩提供大量的热能,促使泥质麻粒岩原岩发生麻粒岩相变质作用,随着岩浆的冷却,泥质麻粒岩受热减少,温度逐降降低,并发生退变质作用(图7)。
6 结论
(1)柴北缘乌兰地区早古生代经南祁连洋向北俯冲所导致的麻粒岩相变质作用,泥质麻粒岩峰期变质时代为(482±3) Ma。
(2)泥质麻粒岩峰期变质P-T条件为710 ℃~730 ℃和5~6.2 kbar,退变质P-T条件为560 ℃~650 ℃和5~6.2 kbar,经逆时针的P-T轨迹。
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The Early Paleozoic Middle-Low Pressure Granulite-Facies Metamorphism: The Evidence from Pelitic Granulite in Wulan Area
Yao Yong1, Yu Shengrao1,2, Ji Wentao1, Li Yan1,Li Zhuofan1
(1.Key Laboratory of Submarine Geosciences and Prospecting Techniques, MOE, Institute for Advanced Ocean Study, College of Marine Geosciences, Ocean University of China, Qingdao,Shandong, 266100, China;2.Laboratory for Marine Geology and Environment, Qingdao National Laboratory for Marine Science and Technology, Qingdao,Shandong,266237, China)
Abstract: The Early Paleozoic middle-low pressure pelitic granulite was exposed in the Wulan area in the northern Qaidam. Petrological and geochronological studies reveal that the Early Paleozoic pelitic granulite in Wulan area underwent two stages metamorphism. The peak mineral assemblage of pelitic granulite is garnet + biotite + sillimanite + K-feldspar +plagioclase + quartz + melt. The retrograde mineral assemblage consists of biotite + muscovite + garnet + H2O ± plagioclase ± quartz. The LA-ICP-MS U-Pb dating of zircons indicate that the granulite-facies metamorphic age is 482 ± 3 Ma. In order to reveal the metamorphic evolutionary history and metamorphic mechanism of the Early Paleozoic pelitic granulite in Wulan area, we conclude the pelitic granulite metamorphic P-T conditions by phase equilibria and the result of Ti-in-biotite thermometer. And the P-T conditions of the peak granulite-facies metamorphism and retrograde metamorphism are 710℃-730℃/5-6.2 kbar and 560℃-650℃/5-6.2 kbar, respectively. Duo to the cooling of magma, the pelitic granulite underwent isobaric cooling metamorphism during retrograde metamorphism.
Key words: The northern Qaidam; Wulan area; Early Paleozoic; Granulite-facies