DISTINGUISHING MAGMATIC FROM SUBSOLIDUS EPIDOTE - LASER PROBE OXYGEN-ISOTOPE COMPOSITIONS

Epidote occurs in four textural varieties in the Mount Lowe intrusion of southern California: euhedra, anhedra, intergrowths and cross-cutti ng veins. Of these, conspicuous elongate euhedra, which range up to 3 cm in length, meet most of the established textural criteria for magma tic epidote. Equant anhedral grains, which are similar to 5 mm in diam eter, are texturally ambiguous although rare allanite cores are sugges tive of a magmatic origin. Epidote intergrowths with hornblende and bi otite also meet certain textural criteria for crystallization from a m agma, notably, euhedral faces against biotite. Finally, late-stage vei ns of epidote cross-cut all phases in the rock and are likely subsolid us. Oxygen isotopic compositions of these four textural varieties of e pidote determined using the laser probe indicate that the use of textu ral criteria alone in establishing epidote parageneses can be misleadi ng. Intracrystalline delta(18)O variations in the euhedra document bot h magmatic and subsolidus compositions. Oxygen isotope compositions ar e bimodal averaging 5.36 +/- 0.13 parts per thousand (n = 11) and 4.66 +/- 0.23 parts per thousand (n = 21). These data combined with plagio clase and zircon delta(18)O values are interpreted to indicate that ma gmatic oxygen isotopic compositions have been preserved where epidote delta(18)O is greater than 5 parts per thousand. Significant portions of each euhedral crystal have been subsolidus exchange and are charact erized by delta(18)O values less than 5 parts per thousand. Intracryst alline delta(18)O values of epidote anhedra range from 4.50 to 6.08 pa rts per thousand and thus also document both magmatic and subsolidus c ompositions. Subsolidus exchange is volumetrically less significant in the anhedra than in the euhedra. Values of delta(18)O for intergrowth s and late-stage cross-cutting veins of epidote average 3.81 +/- 0.22 parts per thousand subsolidus growth. The data confirm that in the Mou nt Lowe intrusion, large euhedra of epidote are magmatic in origin, co nsistent with textural evidence. The data also indicate that equant an hedra of epidote are also magmatic in origin and thus the absence of g ood crystal faces does not necessarily indicate subsolidus growth. The subsolidus origin of intergrowths of epidote with euhedral faces agai nst biotite indicates that well developed crystal faces do not require growth from a magma. Finally, the subsolidus origin of the vein epido te is consistent with textural evidence. The increased resolution avai lable with laser-based oxygen isotope measurements offers an important test for documenting whether epidote is of magmatic or subsolidus ori gin. Given the barometric significance of magmatic epidote, oxygen iso tope compositions can be used to aid in the interpretation of epidote- bearing plutons.