New P–T constraints on the Tamayen glaucophane‐bearing rocks, eastern Taiwan: Perple_X modelling results and geodynamic implications

Abstract

New pseudosection modelling was applied to better constrain the P-T conditions and evolution of glaucophane-bearing rocks in the Tamayen block of the Yuli belt, recognized as the world's youngest known blueschist complex. Based on the predominant clinoamphibole, textural relationships, and mineral compositions, these glaucophane-bearing high-P rocks can be divided into four types. We focused on the three containing garnet. The chief phase assemblages are (in decreasing mode):(Type-II), and amphibole + quartz + albite + epidote + garnet + rutile + hematite + titanite (Type-III). Amphibole exhibits compositional zoning from core to rim as follows: glaucophane ? pargasitic amphibole ? actinolite (Type-I), barroisite ? Mg-katophorite/taramite ? Fe-glaucophane (Type-II), glaucophane ? winchite (Type-III). Using petrographic data, mineral compositions and Perple_X modelling (pseudosections and superimposed isopleths), peak P-T conditions were determined as 13 AE 1 kbar and 550 AE 40°C for Type-I, 10.5 AE 0.5 kbar and 560 AE 30°C for Type-II (thermal peak) and 11 AE 1 kbar and 530 AE 30°C for Type-III. The calculations yield higher pressures and temperatures than previously thought; the difference is~1-6 kbar and 50-200°C. The three rock types record similar P-T retrograde paths with clockwise trajectories; all rocks followed trajectories with substantial pressure decrease under near-isothermal conditions (Type-I and Type-III), with the probable exception of Type-II where decompression followed colder geotherms. The P-T paths suggest a tectonic environment in which the rocks were exhumed from maximum depths of~45 km within a subduction channel along a relative cold geothermal gradient of~11-14°C km À1 .

A reinterpretation of the metamorphic Yuli belt: Evidence for a middle-late Miocene accretionary prism in eastern Taiwan

“…(a) Blueschist with a crystallization age of 15.4~16 Ma by magmatic zircon U‐Pb dating, the spotted schist of a mélange deposits formed after middle Miocene by detrital zircon U‐Pb dating (this study). (b) Peak metamorphism of spotted schist at 486~550°C and 7.4~9 kbar [ Chiang , ] or 530~550°C and 15.5~17 kbar [ Baziotis et al ., ; Keyser et al ., ], amphibolites at ~500°C and 7~8 kbar [ Liou , ], and blueschist at ~550°C and 10~12 kbar [ Beyssac et al ., ]. The Yuli belt inferred to have developed under P‐T conditions of 486~550°C and 7~15.5 kbar.…”

“…However, based on the results of previous geochronological and metamorphic studies, the Yuli belt is inferred to have developed under P‐T conditions of 486~550°C and 7~17 kbar [ Beyssac et al ., ; Chiang , ; Liou , ; Baziotis et al ., ; Keyser et al ., ] and is interpreted as a metamorphosed mélange formed at deeper levels of the accretionary complex during subduction (Figure b). This suggests that the middle‐late Miocene metamorphic complex resulted from subduction of the South China Sea oceanic crust beneath the Philippine Sea Plate before the arc‐continent collision at 6~5 Ma.…”

“…The Yuli belt to the east is composed mostly of spotted schist (Figure a) with enclosed allochthonous blocks of blueschist (Figure b), amphibolite, nephrite, serpentinite (Figure c), gabbro, albitite (Figure d), chlorite schist, and metabasite and has also been inferred to be a Mesozoic mélange [ Biq , ; Jahn and Liou , ; Jahn et al ., ; Liou , ; Liou et al ., ; Lo and Yui , ; Yen , ; Yui and Lo , ; Yui et al ., ]. Moreover, the amphibolite facies metamorphic spotted schist and enclosed blocks of blueschist exhibit amphibolite‐high greenschist facies [ Baziotis et al ., ; Keyser et al ., ]. Jahn et al .…”

“…The Yuli belt consists mainly of spotted schist, including exotic metamafic blocks [ Yen , ] (Figures a and b). This belt is characterized by amphibolite‐upper greenschist facies for the host rocks of spotted schist and blueschist, which record peak temperatures and pressures of about 486~550°C and 7~17 kbar, respectively [ Liou et al ., ; Liou , ; Chiang , ; Beyssac et al ., ; Baziotis et al ., ; Keyser et al ., ]. Previous studies of these rocks have focused on 40 K/ 40 Ar, 40 Ar/ 39 Ar, and 87 Rb/ 86 Sr geochronological dating of exotic metamafic rocks with cooling ages of about 14~8 Ma.…”

Iranshahr blueschist: subduction of the inner Makran oceanic crust

“…Sodic-calcic and calcic amphibole around glaucophane are frequently described for high-P terranes that have undergone isothermal decompression (e.g. Tian & Wei, 2014;Barziotis et al, 2016). Minerals such as chlorite, calcic amphibole, epidote and albite are typical phases of metabasites in greenschist facies metamorphism.…”