Discovery and P-T-t Paths of Lawsonite Pseudomorph-Bearing Eclogites in the Yuka Terrane, North Qaidam Ultrahigh Pressure Metamorphic Belt and Exploration on Key Factors Controlling Lawsonite Formation
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摘要: 硬柱石是大洋冷俯冲带的代表性矿物之一，富含水和Sr、Pb及稀土等微量元素，其形成和分解对于俯冲带流体活动、壳幔水和微量元素循环、地幔楔交代和熔融及岛弧岩浆作用等具有重要影响.但由于硬柱石对温度和压力的改变非常敏感，在板片折返过程中很容易分解，因此目前全球出露的硬柱石榴辉岩极为稀少.总结了榴辉岩中早期硬柱石存在的识别标志，并据此确定柴北缘超高压带西段鱼卡地区的含蓝晶石榴辉岩和斜黝帘石榴辉岩是峰期硬柱石榴辉岩退变质改造的结果.该发现说明柴北缘成为继大别造山带之后全球第二例出露硬柱石榴辉岩的大陆俯冲型造山带.利用相平衡计算方法恢复了这两种榴辉岩的变质演化过程，其中含蓝晶石榴辉岩的P-T轨迹和峰期变质条件均与区内大陆俯冲型含柯石英多硅白云母榴辉岩相似，而斜黝帘石榴辉岩峰期变质温压则略低.锆石定年获得含蓝晶石榴辉岩和斜黝帘石榴辉岩的变质时代分别为437 Ma和436 Ma，与带内已有超高压榴辉岩相变质时代相同，同时获得含蓝晶石榴辉岩的原岩结晶时代为1 273 Ma.相似的变质P-T轨迹和变质时代表明含蓝晶石榴辉岩与同剖面含柯石英多硅白云母榴辉岩共同经历了大陆深俯冲作用.这一结果表明，硬柱石榴辉岩并非大洋冷俯冲带特有，决定榴辉岩中是否出现硬柱石的主要因素是原岩成分和变质条件.在鱼卡地区，榴辉岩的矿物组合中能否出现硬柱石的最主要控制因素是原岩中的Mg含量，由高Mg#的基性岩变质形成的榴辉岩峰期矿物组合中易出现硬柱石.Abstract: Lawsonite is a representative mineral in cold oceanic subduction zone. It contains high water and trace element contents,such as Sr,Pb,and REE,and thus its formation and breakdown greatly influence fluid activity in subduction channel,deep water and trace element recycling,metasomatism and partial melting of mantle wedge as well as arc volcanism. However,lawsonite-bearing eclogite is rarely exposed because lawsonite is very delicate that seldom survives exhumation. In this paper,we summarize the methods to identify former presence of lawsonite in eclogite and determine that clinozoisite eclogite and kyanite-bearing eclogite in the Yuka terrane,North Qaidam UHPM belt (NQUB) are retrograde products of lawsonite eclogite. This makes the NQUB being the second continental subduction-type orogenic belt which contains lawsonite eclogite. Phase equilibrium modelling obtains clockwise P-T paths for these two eclogites. The P-T path and peak metamorphic conditions of the kyanite-bearing eclogite are similar to the coesite-bearing phengite eclogite in the Yuka terrane,whereas the clinozoisite eclogite has slightly lower peak P-T conditions. Zircon dating yields metamorphic ages of 437 Ma and 436 Ma for the kyanite-bearing eclogite and clinozoisite eclogite,respectively,which are similar to UHP eclogite facies metamorphic ages in the NQUB,and a protolith age of 1 273 Ma for the kyanite-bearing eclogite. The similar P-T path and metamorphic age of the kyanite-bearing eclogite with coesite-bearing eclogite indicate that these two eclogites underwent continental deep subduction together. This results show that lawsonite eclogite is not peculiar to oceanic cold subduction zone. The main factors controlling the formation of lawsonite eclogite are bulk composition and metamorphic conditions. In the Yuka terrane,eclogite with high Mg# favors lawsonite-bearing peak assemblage.
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Key words:
- lawsonite pseudomorph /
- eclogite /
- P-T-t path /
- Yuka terrane /
- North Qaidam /
- pertology
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图 1 斜黝帘石榴辉岩(a~c)及含蓝晶石榴辉岩(d)中的硬柱石假象
Grt.石榴石；Omp.绿辉石；Ph.多硅白云母；Amp.角闪石；Pg.钠云母；Czo.斜黝帘石；Ky.蓝晶石；Pl.斜长石；Ttn.榍石；Qtz.石英
Fig. 1. The lawsonite pseudomorphs in the clinozoisite eclogite (a-c) and kyanite-bearing eclogite (d)
图 2 鱼卡不同榴辉岩的P-T轨迹
Ec.榴辉岩相；GR.麻粒岩相；AM.角闪岩相；EA.绿帘角闪岩相；BS.蓝片岩相；GS.绿片岩相
Fig. 2. The P-T paths for eclogites in theYuka terrane
图 3 鱼卡不同榴辉岩球粒陨石标准化稀土图(a)及原始地幔标准化蛛网图(b)
数据引自Chen et al.(2009)；Song et al.(2010)；Ren et al.(2017, 2018)
Fig. 3. Chondrite-normalized REE diagram (a) and primitive mantle-normalized trace element diagram (b) for Yuka eclogites
图 4 斜黝帘石榴辉岩(a)及含蓝晶石榴辉岩(b)锆石定年结果
Fig. 4. Zircon dating results of the clinozoisite eclogite (a) and kyanite-bearing eclogite (b)
图 5 全岩Mg#对鱼卡榴辉岩峰期矿物组合的影响
Lws.硬柱石；Tlc.滑石；Chl.绿泥石；Hbl.普通角闪石；Ep.绿帘石；Coe.柯石英；其余同图 1；据Ren et al. (2018)
Fig. 5. P-T-Mg# pseudosections showing the peak metamorphic mineral assemblage controlled by bulk Mg#
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