Petrological properties and tectonic significance for Longyou garnet amphibolite

Abstract

In view of the recent issue proposed by some researchers that "the garnet amphibolite in Longyou county of Zhejiang was retrograded from Caledonian collision-related eclogite". Based on detailed field geological investigation, we carried out a systematic study including petrography, mineral chemistry, zircon geochronology and bulk rock geochemistry. Our results indicate that, the relatively small amounts of plagioclase which only found in interior of garnet or clinopyroxene metacryst are anorthite, while plagioclase that largely distribute in the outer part of garnet or clinopyroxene metacryst are andesine and labradorite (with their An value within 44-66), and no sodium-rich plagioclase (albite or oligoclase) which imply omphacite decomposition structure are found. The CL images of zircon exhibit obvious core-rim structure. U-Pb dating results indicate that the weighted average age of the zircon core part is 879±11 Ma, with Th/U ratio values ranging from 0.20-1.23 which suggest their magmatic origin; while the age of the rim is 446±5 Ma, and lots of their corresponding Th/U ratio show low values (≤0.1), suggesting their metamorphic origin. Moreover, the bulk rock geochemistry feature of garnet amphibolite suggest that its protolith is basic (the SiO2 content within 46.8%-48.2%), and its trace element exhibit E-MORB geochemical affinity. Based on the effective bulk rock composition and EPMA analysis results, as well as the mineral association we found, the P-T pseudosection for Longyou garnet amphibolite was worked out. And this figure reveals that the Longyou garnet amphibolite at least experienced three stages of metamorphism: the prograde metamorphism stage with P-T condition of 0.67-0.75 GPa/685-719℃; the peak-metamorphism with P-T condition of 0.91-0.97 GPa/766-782℃; and the retrograde metamorphism stage with P-T condition of ~0.68 GPa/~767℃ (by traditional P-T thermobarometer, we also get a P-T condition of 0.66-0.67 GPa/720-740℃ for retrograde metamorphism stage). Whichever stage, its pressure is medium-pressure but not high-pressure. In combination with regional geological setting, we prefer the following genetic model for Longyou garnet amphibolite: the paleo-South China Ocean subduction under Yantze plate in Neoproterozoic→trigger back-arc extension→mantle-derived magma upwelling and forming E-MORB type basic rocks in Neoproterozoic→medium to high-grade metamorphism in Silurian.