Metamorphism during Ophiolite Emplacement the Petrology of the St. Anthony Complex

The St. Anthony Complex consists of the White Hills Peridotite(the ultramafic remnant of an ophiolite), and an underlyingmetamorphic aureole which grades from metagabbro and hornblendegranulite through amphibolite, epidote amphibolite, and greenschistinto undeformed volcanic rocks at the base of the sequence.The petrology and mineralogy of the complex show that the basalperidotite mylonites recrystallized at 900–950 °C,the coronitic metagabbros at 850–900 °C, two-pyroxeneamphibolites at 860 °C, marbles at 680 °C, epidote amphibolitesat 550–650 °C, and greenschists at 350–550 °C.The metamorphic pressures range from 7–10 kb at the peridotitecontact to 3–5 kb in the greenschists and amphibolites,and less than 2 kb in the volcanics. The geology suggests thatthe metamorphic rocks originally formed from a strip of turbidites,alkali basalts, tholeiites, and gabbros adjacent to a continentalmargin. This sequence was subjected to dynamothermal metamorphismresulting from a combination of conduction of heat from theoverlying ultramafic rocks and shear heating. The rocks weregradually accreted onto the base of the overriding ophiolitesheet, forming the composite metamorphic sequence now observed.