Michael Bröcker

New Age Constraints on the Metamorphic Evolution of the High-Pressure/Low-Temperature Belt in the Western Tianshan Mountains, NW China

The western Tianshan high‐pressure/low‐temperature orogenic belt in NW China contains eclogite‐facies metavolcanic rocks and omphacite‐bearing blueschists. Previous Sm‐Nd (omphacite, garnet, glaucophane, whole rock) and 40Ar/39Ar (crossite) dating of eclogite‐facies rocks has suggested an age of ca. 345 Ma as the best approximation for the timing of peak metamorphic conditions. The samples described here are blueschist‐facies rocks that formed during or after the transition from the eclogite‐facies to the epidote‐blueschist‐facies and subsequently experienced an incipient greenschist‐facies overprint. By use of white mica geochronology (K‐Ar, 40Ar/39Ar, Rb‐Sr), an attempt is made to date postpeak metamorphic stages of the complex PT path. Rb‐Sr and 40Ar/39Ar ages range between 313 and 302 Ma and 323 and 312 Ma, respectively, but mostly cluster at ca. 310–311 Ma, indicating that the studied samples recrystallized at this time. However, the 40Ar/39Ar age spectra show complex release patterns that are interpreted to be influenced by excess argon to varying degrees. This conclusion is further corroborated by K‐Ar dates ranging between 385 and 309 Ma. Younger dates of ca. 302 Ma (Rb‐Sr) and ca. 296 Ma (K‐Ar) indicate subsequent disturbances of these isotope systems in some rocks. The new ages are significantly younger than the time of eclogite‐facies metamorphism (350–345 Ma), indicating resetting of the 40Ar/39Ar, K‐Ar, and Rb‐Sr systems during exhumation of the blueschist‐facies rocks. Furthermore, this dataset suggests that high‐pressure conditions were attained during the Carboniferous and not at Permian or Triassic time, as recently suggested by SHRIMP U‐Pb zircon dating.

Rb-Sr isotope studies on Tinos Island (Cyclades, Greece): additional time constraints for metamorphism, extent of infiltration-controlled overprinting and deformational activity

This study presents new Rb-Sr age data concerning the metamorphic evolution of the Attic- Cycladic Crystalline Belt which represents a complex polymetamorphic terrane within the Alpidic orogenic belt of the Hellenides. Two major groups of tectonic units can be distinguished. Metamorphism in parts of the upper units is commonly considered as a Cretaceous event. In contrast, the group of lower units experi- enced Tertiary high-pressure metamorphism which was followed by a medium-pressure overprint. We focus on the island of Tinos where a representative spectrum of the rock units found in the Cyclades is exposed in three tectonic units: the Upper Unit, the Intermediate Unit and the Basal Unit. The complete range of tectono-metamorphic and magmatic events affecting the Attic-Cycladic Crystalline Belt is documented by numerous petrological and tectonic studies. Phyllites and phyllonites from the ophiolitic Upper Unit yielded Rb-Sr apparent ages (phengite-whole-rock) between c. 92 and 21 Ma. The older age differs from the Cretaceous dates reported for upper unit rocks elsewhere in the Cyclades. It is suggested that the sequence studied belongs to the Jurassic ophiolites of the Hellenides rather than to Cretaceous occurrences. The spread to younger ages is related to non-pervasive rejuvenation and resetting of the Rb-Sr system during tectonic juxtaposition of the Upper Unit over the Intermediate Unit. The youngest age obtained so far for a sample from the Upper Unit (21 Ma) is believed to approximate the timing of tectonic juxtaposition which probably occurred during a regional greenschist-facies episode producing a pervasive overprint in the struc- turally lower tectonic unit. The major phyllite/meta-gabbro/serpentinite sequence of the Upper Unit is inter- preted as an emplacement-related ductile shear zone which experienced reworking under brittle conditions. In the Intermediate Unit, a gradient in Rb-Sr ages from top (c. 40 Ma) to the bottom (c. 22 Ma) was recog- nized, which is interpreted to represent greater effects of fluid infiltration and overprinting in the lower parts of this unit, possibly controlled by variable intensity of deformation which might be related to tectonic jux- taposition onto the Basal Unit. We suggest that synmetamorphic stacking of all three tectonic units took place during an Oligocene-Miocene greenschist event. Extensional deformation continued after tectonic stacking and after intrusion of the main granite, as is indicated by a Rb-Sr whole-rock isochron (15.1 ± 0.6 Ma) for a ductilely deformed garnet-bearing leucogranite from the marginal parts of the main undeformed pluton. Application of the Rb-Sr dating technique provided no unequivocal evidence that previously pub- lished Eocene K-Ar and 40 Ar- 39 Ar dates for high-pressure phengites from the lower units are significantly contaminated with excess argon.

U-Pb zircon geochronology of unusual eclogite-facies rocks from Syros and Tinos (Cyclades, Greece)

Low-temperature eclogite-facies rocks from Syros and Tinos (Cyclades, Greece) include meta-igneous blocks of unclear origin (meta-olistostrome or tectonic melange) with very high trace element concentrations (e.g. Zr up to 4850 ppm; Y up to 475 ppm). The unusual geochemistry is considered to be the result of metasomatic alteration in a subduction-zone environment. Locally, metasomatic exchange with an ultramafic matrix further enhanced compositional anomalies. A concordant U-Pb zircon age of 78 ∠ 1 Ma recorded for an omphacitite from Syros is interpreted as the time of pre-Eocene high-pressure metamorphism in the Cyclades. Zircon dates of 61 and 63 Ma for a jadeitite from Tinos possibly indicate an additional discrete event (deformation?). These results are a first geochronological indication that high-pressure metamorphism in the Cyclades commenced signif- icantly earlier than indicated by previous white mica chronology which provided ages between c. 50 and 40 Ma.

Fluid–metasedimentary rock interactions in subduction-zone mélange: Implications for the chemical composition of arc magmas

Elevated concentrations of certain large ion lithophile elements (LILE; e.g., Ba, K, Rb, Cs, Ca, Sr), U, and Pb in arc magmas relative to high field strength elements (HFSE; e.g., Ti, Th, Hf, Nb, Zr) are considered key indicators of fluid addition to arc magma source regions worldwide, but the fluid sources and processes of mass transfer are controversial. Dehydration of downgoing slabs releases fluids that can flow through and react with meta- morphosed ultramafic-mafic rock packages in melange zones near slab-mantle interfaces. New geochemical data from Syros, Greece, reveal that these fluids preferentially leach LILEs, U, and Pb when they infiltrate and react with subducted metasedimentary rocks. Transfer of these LILE-, U-, and Pb-enriched fluids to the mantle wedge at subarc depths could directly trigger partial melting and generate magmas with elevated Ba/Th, Sr/Th, Pb/Th, and U/Th, as well as radiogenic Sr. Alternatively, if fluid transfer occurs at shal- lower depths (e.g., Syros), the metasomatized mantle could be carried deeper by wedge corner flow to ultimately undergo partial melting in subarc regions.

Fast extension but little exhumation: the Vari detachment in the Cyclades, Greece

Markedly different cooling histories for the hanging- and footwall of the Vari detachment on Syros and Tinos islands, Greece, are revealed by zircon and apatite fission-track data. The Vari/Akrotiri unit in the hangingwall cooled slowly at rates of 5-15 ◦ CM yr −1 since Late Cretaceous times. Samples from the Cycladic blueschist unit in the footwall of the detachment on Tinos Island have a mean zircon fission-track age of 10.0 ± 1.0 Ma, which together with a published mean apatite fission-track age of 9.4 ± 0.5 Ma indicates rapid cooling at rates of at least ∼ 60 ◦ CM yr −1 .W e derive am inimum slip rate of ∼ 6.5 km Myr −1 and a displacement of >∼ 20 km and propose that the development of the detachment in the thermally softened magmatic arc aided fast displacement. Intra-arc extension accomplished the final ∼ 6-9 km of exhumation of the Cycladic blueschists from ∼ 60 km depth. The fast-slipping intra-arc detachments did not cause much exhumation, but were important for regional- scale extension and the formation of the Aegean Sea.

Unusual bulk-rock compositions in eclogite-facies rocks from Syros and Tinos (Cyclades, Greece): implications for U–Pb zircon geochronology

Low-temperature eclogite-facies rocks from Syros and Tinos (Cyclades, Greece) include meta-ophiolitic blocks of unclear origin (meta-olistostrome or tectonic melange). These blocks occur in a matrix of altered serpentinite and/or semipelitic to tuffitic schists. Some samples are characterized by very high trace-element concentrations, for example, eclogites from Tinos contain up to 4950 ppm Zr and up to 480 ppm Y. Understanding of the geochemical controls on the petrogenesis of these rocks has important consequences for interpretation of geochronological results. The emphasis of this study is on the behaviour of Zr under eclogite-facies P–T conditions. Due to remarkably high modal amounts of zircon, some samples from the meta-ophiolite suite are suitable for U–Pb zircon chronology. The geological significance of zircon ages can only be correctly interpreted by assessing whether zircon crystallized from a melt or formed during subsequent hydrothermal or metamorphic processes. The results of this study lead us to conclude that infiltration of trace-element enriched fluids during eclogite-facies metamorphism caused local-scale compositional changes. Textural observations document the presence of a fracture network favouring fluid infiltration under high-pressure conditions. Such a system of microfractures might have provided infiltration paths for trace-element enriched fluids during earlier stages of high-pressure metamorphism. In most cases, subsequent recrystallization erased any evidence of the early infiltration channels, but inclusion of high-pressure phases in zircon provide evidence for this model, suggesting metasomatic alteration in a subduction-zone environment. The original fluid source and the conditions (e.g., high-F environment, highly saline brines?) under which commonly immobile elements were mobilized remain unclear. It is speculated that eclogitization of closely associated metagabbros may have released Zr and other trace elements. The results of this contribution suggest that new zircon formed during high-pressure metamorphism. Cretaceous U–Pb zircon ages are interpreted to date subduction metamorphism and not magmatic crystallization.

Protolith Ages of Meta‐igneous and Metatuffaceous Rocks from the Cycladic Blueschist Unit, Greece: Results of a Reconnaissance U‐Pb Zircon Study

The Cycladic blueschist unit (CBU) in the central Aegean Sea comprises a pre‐Alpidic crystalline basement that is overlain by thrust sheets of a metamorphosed volcanosedimentary sequence. Mélanges are widespread and enclose a wide variety of meta‐igneous rock fragments in a serpentinitic and/or metasedimentary matrix. Previous geochronology has documented the general temporal framework for the Alpidic metamorphic history but did not produce well‐constrained protolith ages for the main volcanosedimentary succession and for mélange blocks. This issue is addressed here in an ion probe U‐Pb zircon study focusing on the islands of Andros, Sifnos, and Ios. The new results indicate a consistent regional pattern of Triassic ages (ca. 237–245 Ma) for the magmatic precursors of metatuffaceous and metavolcanic rocks, which occur as intercalations within metasediments. This narrow range of igneous crystallization ages records an important period of volcanic activity in the larger study area. On Andros, such ages were also determined for a disrupted tabular body of meta‐acidic gneisses (several hundred meters in length) that has been considered an olistolith within a meta‐olistostrome. Field observations are compatible with the interpretation that this fragment represents a layer of the main succession that has been disrupted by large‐scale boudinage. A metagabbro and a meta‐acidic gneiss from a high‐pressure mélange on Andros yielded Jurassic 206Pb/238U ages of \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape

Ultrahigh-Pressure Metamorphism in the Śnieżnik Mountains (Sudetes, Poland): P-T Constraints and Geological Implications

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