Metamorphosed evaporites

Evaporite salts can survive well into the metamorphic realm, but are altered, recrystallised or transformed into new minerals. And so, beyond the early greenschist phase, little or nothing of the original sedimentary mineral phase remains with the possible exception of anhydrite (Table). Consider “the before and after” situation as a sequence of evaporite-entraining sediments passes into the metamorphic realm. Under a likely “before” regional metamorphism scenario, a typical buried marine evaporite body occupies a continental margin position, is hundreds of meters thick, tens to hundreds of kilometers wide, perhaps with halokinetic geometries and an extensively tilted and faulted overburden. Halite typically makes up more than 60-80% of the total rock salt volume in a basinwide unit.. This halite is likely to be interlayered with anhydrite, bitterns, magnesian carbonates and siliciclastic clays. “After” metamorphism, typically driven by subduction and continent-continent collision, the end product of this once dominantly NaCl body of rock is a series of sodic and magnesian aluminosilicates, magnesites, calc-silicates and marbles with local zones of potassic enrichment. The immediate question is, where did all that salt and the associated volatiles (Cl, CO2, SO3) go?

 

Some salt components may still reside in meta-evaporite indicator minerals, such as sodic or magnesian talc, lazurite, marialitic scapolites or tourmalinites . The remainder went elsewhere, perhaps as the transforming evaporite mass generated metalliferous volatile-entraining brines. The presence of volatiles and their mobilization in metamorphic transformation is typical of meta-evaporite associations. A relatively open system of hydrofractures and shear strength contrasts in the form of fault/shear-related fluid/volatile conduits was already present in the evaporitic protolith and encouraged by the polyhedral transformation of the NaCl lattice as it enters the greenschist pressure realm.

 

Such a “leaky” prognosis is a somewhat different subsurface scenario from the classic isochemical paradigms and closed chemical system assumptions that permeate much of the older metamorphic literature. More recent work has challenged and displaced the classic “no loss” and homogeneity assumptions in metamorphic mineral phase evolution. This is especially so when evaporites enter the contact, dynamic, hydrothermal and regional metamorphic realms (warren, 2016; Chapter 130.

 

 

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