US EPA

6863228 

Journal Article 

MISSING MINERAL ZONES IN CONTACT METAMORPHOSED BASALTS 

Manning, CE; Ingebritsen, SE; Bird, DK; , 

1993 

Yes 

American Journal of Science
ISSN: 0002-9599 

KLINE GEOLOGY LABORATORY 

NEW HAVEN 

894-938 

WOS:A1993MG72700002 

We describe an extreme example of the kinetic controls on the contact metamorphism of basalts in which a predicted mineral zone is missing. In the metabasalts that host the Skaergaard intrusion in East Greenland, a high-grade pyroxene hornfels (the pyroxene zone) is in contact with a low-grade greenschist-facies mineral assemblage (the actinolite + chlorite zone). Hornblende-bearing assemblages are absent, and there is no evidence that they ever formed. The boundary between the two mineral zones occurs at 250 m from the contact and records an abrupt change in metamorphic temperature as well as mineral assemblage and texture. Mineral assemblages for the high-grade rocks reflect peak metamorphic temperatures that decreased from similar to 900 degrees C at the contact to similar to 800 degrees C at 250 m. Beyond this distance, the greenschist-facies mineral assemblages replacing relict igneous minerals suggest temperatures of 300 degrees to 550 degrees C. Numerical simulations of heat and fluid flow for a model approximating the geometry of the Skaergaard magma-hydrothermal system reproduce the peak temperatures observed within 100 m of the contact only for basalt permeabilities of less than or equal to similar to 10(-16) m(2). Whole-rock delta(18)O values suggest cumulative fluid fluxes that require basalt permeability greater than or equal to 10(-16) m(2). The thermometry and isotopic data thus combine to constrain permeability to have been similar to 10(-16) m(2). The abrupt change in metamorphic temperatures inferred from the mineral assemblages was not predicted in the simulations, implying that the 250 m boundary was caused by reaction kinetics. If metamorphic reactions had proceeded such that chemical equilibrium was approached closely, the formation of hornblende should have occurred at similar to 550C. For a permeability of 10(-16) m(2), this temperature was overstepped by similar to 80 degrees C at 250 m when peak temperatures were attained in the simulations. The extent of oxygen isotope alteration suggests transport processes were not rate limiting, and dissolution of the most refractory mineral, clinopyroxene, would have been too rapid to explain the missing mineral zone. Using equations describing temperature oversteps required to achieve observable nucleation rates, we show that the low entropy changes of model metamorphic reactions involving hornblende formation imply that large oversteps of hornblende nucleation reactions are possible and that hornblende nucleation was rate limiting. Missing mineral zones appear to characterize basaltic contact aureoles where the protolith was unaltered, and metamorphism was not progressive.