LASS U–Th–Pb monazite and rutile geochronology of felsic high-pressure granulites (Rhodope, N Greece): Effects of fluid, deformation and metamorphic reactions in local subsystems
LASS U–Th–Pb monazite and rutile geochronology of felsic high-pressure granulites (Rhodope, N Greece): Effects of fluid, deformation and metamorphic reactions in local subsystems
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DOI：
10.1016/j.lithos.2015.06.029
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发表时间：
2015-09
期刊：
影响因子：
3.5
通讯作者：
N. Wawrzenitz;A. Krohe;I. Baziotis;E. Mposkos;A. Kylander‐Clark;Rolf L. Romer
中科院分区：
文献类型：
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作者：
N. Wawrzenitz;A. Krohe;I. Baziotis;E. Mposkos;A. Kylander‐Clark;Rolf L. Romer
剪切带独居石的特定化学成分受造岩矿物同变形溶解-沉淀反应的控制。这种关系可用于测年变形，即使当微组构的特点，如形状优选取向或晶内变形的独居石本身是失踪。在剪切带内和剪切带周围同时形成的独居石可能具有不同的成分。这取决于当地的化学背景，而不是反映连续结晶episodes的独居石。这是证明在多变质，糜棱岩高压（HP）石榴石蓝晶石麻粒岩的阿尔卑斯Sidironero复杂（Rhodope超高压地形，北方希腊）。所研究的糜棱岩在40-36 Ma的区域混合岩化和随后的剪切通过冷却，直到36 Ma逃脱。利用激光原位烧蚀分流电感耦合等离子体质谱（LASS）技术，结合金红石的U-Pb数据和黑云母的Rb-Sr数据，对来自微尺度剪切带、前糜棱岩中的独居石以及残留矿物中的独居石包裹体进行了分析，确定了中生代和新生代两个主要变质期。四个不同微组构域（I-IV）独居石的化学成分、同位素特征和表观年龄存在系统的差异。在三个前糜棱岩域（（I）前糜棱岩蓝晶石和（II）石榴石碎斑岩中的包裹体，以及（III）前糜棱岩细岩中的包裹体）独居石产生的年龄约为1000年。130-150 Ma的高压麻粒岩变质作用，与该地区以往的年代学结果一致。粒内溶解-沉淀作用对前糜棱岩型独居石的斑片状蚀变作用使Eu负异常增加，HREE含量降低。同糜棱岩剪切带（IV）中的独居石与三个前糜棱岩域中的未蚀变独居石和蚀变独居石在化学成分上存在显著差异，表现为Eu负异常明显，HREE模式平坦，Th含量高。这些成分特征与角闪岩相条件下剪切带中斜长石的同糜棱岩形成和石榴石的再吸收有关。剪切带中没有前糜棱岩独居石，与其他域相反，表明旧独居石的完全溶解和新独居石的形成。这可能是由于流体的碱度和反应性的增加，而流体的碱度和反应性又是由剪切带中的长石和磷灰石与同糜棱岩的相互作用控制的。在那里，变形是由溶解沉淀蠕变约。690 ± 50 °C和6-7.5 kbar。在55 ± 1 Ma时独居石的生长确定了这一变形的年代，它先于Sidironero杂岩的区域混合岩化作用，而金红石和黑云母年龄反映了这些后期阶段。这种新的压力-温度-时间约束的残余变形结构提供了洞察仍然缺少的部分的整体变质，变形和折返过程中的超高压单位在罗多彼。
The specific chemical composition of monazite in shear zones is controlled by the syndeformation dissolution–precipitation reactions of the rock-forming minerals. This relation can be used for dating deformation, even when microfabric characteristics like shape preferred orientation or intracrystalline deformation of monazite itself are missing. Monazite contemporaneously formed in and around the shear zones may have different compositions. These depend on the local chemical context rather than reflecting successive crystallization episodes of monazite.This is demonstrated in polymetamorphic, mylonitic high-pressure (HP) garnet–kyanite granulites of the Alpine Sidironero Complex (Rhodope UHP terrain, Northern Greece). The studied mylonitic rocks escaped from regional migmatization at 40–36 Ma and from subsequent shearing through cooling until 36 Ma. In-situ laser-ablation split-stream inductively-coupled plasma mass spectrometry (LASS) analyses have been carried out on monazite from micro-scale shear zones, from pre-mylonitic microlithons as well as of monazite inclusions in relictic minerals complimented by U–Pb data on rutile and Rb–Sr data of biotite.Two major metamorphic episodes, Mesozoic and Cenozoic, are constrained. Chemical compositions, isotopic characteristics and apparent ages systematically vary among monazite of four different microfabric domains (I–IV). Within three pre-mylonitic domains (inclusions in (I) pre-mylonitic kyanite and (II) garnet porphyroclasts, and (III) in pre-mylonitic microlithons) monazite yields ages of ca. 130–150 Ma for HP-granulite metamorphism, in line with previous geochronological results in the area. Patchy alteration of the pre-mylonitic monazite by intra-grain dissolution–precipitation processes variably increased negative Eu anomaly and reduced the HREE contents. The apparent age of this altered monazite is reduced.Monazite in the syn-mylonitic shear bands (IV) differs in chemical composition from unaltered and altered monazite of the three pre-mylonitic domains by having a significantly more pronounced negative Eu anomaly, a flatter HREE pattern, and high Th content. These compositional characteristics are linked with syn-mylonitic formation of plagioclase and resorption of garnet in the shear bands under amphibolite facies conditions. The absence of pre-mylonitic monazite in the shear zones, in contrast to the other domains, suggests complete dissolution of old and formation of new monazite. This probably results from an increased alkalinity and reactivity of the fluid that again is controlled by syn-mylonitic interaction with feldspar and apatite in the shear zones. There, the deformation was accommodated by dissolution precipitation creep at ca. 690 ± 50 °C and 6–7.5 kbar. Growth of monazite at 55 ± 1 Ma dates this deformation, which precedes the regional migmatization of the Sidironero Complex, whereas rutile and biotite ages reflect these later stages. This new pressure–temperature–time constraint for a relictic deformation structure provides insight into the still missing parts of the overall metamorphic, deformation and exhumation processes of the UHP units in the Rhodope.