Abstract

The Big Maria Mountains of southeastern California comprise a Late Cretaceous regional metamorphic terrain involving Paleozoic cratonal sediments. Siliceous limestone of the upper Paleozoic Supai Formation has reacted to form massive wollastonite, requiring an enormous fluid flux. The minimum volume ratio of fluid:rock that is necessary to explain the formation of wollastonite may be calculated from the reaction quartz + calcite = wollastonite + CO2, using the method of Rice and Ferry. Given average conditions of 3 kbar, 500 °C, an infiltrating fluid of composition XH2O = 1.00, an equilibrium fluid composition of XH2O = 0.97, and 90% wollastonite in the final rock, a fluid:rock ratio of 17:1 may be calculated. Infiltrating fluids of composition 1.00 > XH2O > 0.97 require still higher ratios. Metamorphic reactions which took place in other units of the sequence (metapelites, massive carbonates, metavolcaniclastics) did not record the massive fluid flux, but fluids must have passed through them to have affected structurally isolated masses of Supai Formation. Passage of fluids must have occurred along fractures.

Neither magmatism nor radioactive heat sources are adequate to explain the temperatures of metamorphism. If the minimum quantity of fluid that is estimated to have passed through the area was initially at 680 °C, it would result in a 300-degree rise in temperature over that of the stable-craton geotherm. Late metamorphic pegmatite dikes which are most abundant in areas of high metamorphic grade may stem from melts anatectically derived at deeper levels, perhaps as a result of the same fluid flux. Heat that is introduced by large fluid fluxes may be an important cause of anomalously high temperatures which are observed in many cases in regional metamorphic terrains.

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