Hypersaline fluids in Precambrian deep-crustal metamorphism

Abstract

An outstanding debate about the evolution of Precambrian granulite facies terranes concerns the role of fluids in deep-crustal metamorphism. One line of thought ascribes dryness and large-ion element (LILE) depletion to dehydration melting of rocks containing biotite and amphibole, without important participation of a low-density fluid phase, and removal of the granitic melts to the upper crust during metamorphic episodes. This mechanism is supported for the highest-temperature granulite terranes by experimental rock-melting studies. An alternative approach emphasizes field evidence for alkali metasomatism and silica mobility on outcrop and thin-section scales, which seemingly demonstrate fluid-driven processes. The debate concerns whether or not fluids could have coexisted with partial melts in migmatitic terranes and have been important agencies in destroying silicate hydrates and transporting LILE, especially Rb and U, out of the lower crust. Nearly ubiquitous CO2-rich fluid inclusions in granulite facies rocks have been cited by many workers as evidence that important granulite facies fluids were carbonic, in contrast to the H2O-dominated fluids of lower-grade metamorphism. However, the low solubilities of silicate constituents in CO2-rich fluids and the low wetting ability (high dihedral angles) of such fluids relative to silicate mineral grain boundaries, inhibiting infiltration, have been revealed in experimental studies over the last two decades. These features seriously detract from the appeal of carbonic fluids as important agencies in deep-crustal metamorphism. Recent experimental work in the system NaClKclH2O at deep crustal temperatures and pressures demonstrates that concentrated brines have appropriate low H2O activity, high infiltration ability, and high alkali mobility (especially Rb affinity). Limited anatexis may be promoted by hyperfusible solutes such as F and B, but the low H2O activity restricts rock melting. Improved thermodynamic properties for biotite indicate that the H2O activities necessary for orthopyroxene stability in quartzofeldspathic rocks, in lieu of biotite, are considerably higher than estimates based on previously available data, and that brines which participate in granulite facies metamorphism at deep-crustal conditions could be only moderately concentrated, similar in character to those of fluid inclusions observed in many different kinds of rocks. Recent observations of brine inclusions in granulites support the concept that polyionic salt solutions, immiscible with CO2 in the high-grade metamorphic temperature range (700–850°C) are feasible granulite facies fluids. The importance of hypersaline fluids in high-grade metamorphic processes may have been greatly underestimated from previous fluid inclusion studies. Of the several conceptual sources of hypersaline fluids in the crust, volatile-rich alkaline basalts seem plausible because of their additional ability to deliver heat for metamorphism. The postulated magmatic emanations may split into concentrated brines and immescible CO2 during ascent. They transport alkalis and LILE upward in the crust. Alkalic basalts in modern continental settings are characteristic of oblique plate convergence or continential distension, and may represent remelting of subcontinental mantle previously enriched in volatiles and alkalis by subduction processes. A possible Phanerozoic example is the interior western U.S.A. This analogy leads to a ‘Basin and Range’ hypothesis (Hopgood and Bowes, 1990, Tectonophysics 174, 279–299), in which the ancient granulite facies terranes represent zones of cratonal deformation. Granulites of the uplifted and deeply eroded Precambrian mobile belts may record zones of extraordinary thermal and physicochemical activity effected by primitive plate tectonic processes involving continental collision and disruption.