Low-grade metamorphism of the Takitimu Group, western Southland, New Zealand

Publisher Website
Google Scholar
Abstract
A 14000 m homoclinal sequence of Lower Permian rhyodacitic to basaltic volcanic, pyroclastic, and epiclastic horizons and shallow intrusives in the Takitimu Mountains, New Zealand, is described. Two metamorphic zones are recognised. Zone 1 is characterised by the phases laumontite, epistilbite, prehnite, yugawaralite, analcime, stilbite, chabazite, heulandite, celadonite, calcite, and chlorite. Zone 2 is characterised by prehnite, pumpellyite, epidote, albite, chlorite, actinolite, calcite, and less commonly andradite. Electron microprobe analyses are presented for many of these minerals. Zone 1 (zeolite facies) assemblages crop out in the eastern Takitimu Mountains stratigraphically above Zone 2 (prehnite-pumpellyite facies) assemblages. Metamorphism has produced mono- or bimineralic zones (or metadomains) in and adjacent to void space in the rocks, leaving the host rock enriched in albite and chlorite. The metadomains show a subdivision into type A: vesicular tops of flows, void space in permeable strata and small fractures, and type B: major fractures and shear zones. In Zone 1, type A is characterised by zeolites ± calcite and type B by prehnite-quartz. In Zone 2, prehnite-quartz occupies metadomains of type A, and epidote-quartz occupies metadomains of type B. On a regional scale, mineralogical adjustment relates to increasing PLOAD and T increasing depth of burial. Mesoscopically, on outcrop scale, contrasting assemblages in metadomains of types A and B can be explained in terms of varying permeability, and In Zone 1, changes in zeolite assemblages occurring on a microscopic scale are interpreted on the basis of variations in fluid chemistry. In some volcanic arc terrains, short-lived, high temperature gradients related to subvolcanic plutonism may result in a sequence of metamorphic assemblages equivalent to the low-pressure facies series of Seki. The metamorphism of the Takitimu volcanic pile and similar terrains may be essentially plurifacial with initial short-lived, high thermal gradients followed by a longer period of burial, after plutonism, under a moderate geothermal gradient.