MINERALOGY OF CLASTS AND MATRICES IN A SILURO-ORDOVICIAN TECTONIC MéLANGE: NEW CONSTRAINTS AND NEW COMPLICATIONS ON PRE-ACADIAN TECTONIC EVOLUTION IN SOUTHERN NEW ENGLAND

A recently identified tectonic mélange zone occupies an arcuate NE-SW-trending sliver of the eastern Orange Milford Belt in south-central Connecticut . The mélange contains two distinct units: 1) a serpentine-carbonate schist containing ultramafic mineral and lithic fragments; and 2) a polymict argillaceous metaconglomerate. Rock samples were analyzed by powder X-ray diffraction, and quantitative mineral modes were obtained by the Rietveld method. Clasts and surrounding matrices were analyzed separately to identify exotic and native clasts and to discriminate inherited grains from minerals grown during regional Devonian chlorite-grade metamorphism.

The serpentinite contains fragments of grey-white diopside megacrysts up to 40 cm across and clasts of serpentinite 1 mm to >10 m across. These are enveloped by a strongly foliated matrix of carbonate (calcite and/or dolomite) + serpentine + chlorite + magnetite. The matrix-supported metaconglomerate includes subangular to rounded schistose clasts (~1-5 cm). These clasts contain chlorite and garnet in abundances 2-6 times greater than those of their surrounding matrix. Some clasts contain >20% paragonite and minor amounts of epidote, minerals which are present at or below detection in matrix analyses. The strongly foliated muscovite + chlorite + quartz matrix contains minor but systematically higher concentrations of plagioclase and hematite than the clasts. No sample has been observed to contain both serpentine and muscovite. Randomly oriented chlorite porphyroblasts, some >1 cm, composing as much as 10% of the matrix, may represent the equilibration product of these components during Acadian lower greenschist facies metamorphism.

These results suggest the mélange zone was derived from an exhuming metamorphic belt that included high-pressure and ultramafic units erased from the geologic record elsewhere. The pre-Acadian fabric of the MLC greenstones is defined by amphiboles with compositional zonation characteristic of growth during prograde greenschist to epidote-amphibolite facies conditions. However, high-pressure minerals (e.g., glaucophane) have not been identified. The MLC and the provenances of the different mélange units may thus belong to different crustal blocks across a Silurian or even Ordovician suture.