Temperature-Induced Aluminum Zoning in Hornblendes of the Fish Canyon Magma, San Juan Volcanic Field, Colorado

An extensive electron microprobe survey of amphibole compositions in the Fish Canyon magma (2146 analyses), more than 80% of which are from high-resolution (<10 mm step) core-to-rim traverses across large euhedral phenocrysts, provides: (1) temporal constraints on the immediately pre-eruptive P-T evolution of the magma, and (2) a means of testing recent calibrations of Al-in-hornblende thermo-barometry. The low-variance phase assemblage of the Fish Canyon magma (11 mineral phases + melt +/- vapor) is ideal for an assessment of the influence of P and T on hornblende chemistry, particularly as it has been reproduced experimentally at X_H2O = 0.5, 760° C, 2.4 kb and f_O2 = -11.4 by Johnson and Rutherford (1989; JH89). Hornblende phenocrysts are variable for most major elements (e.g., 5-9 wt.% Al₂O₃), due primarily to two T-sensitive coupled substitutions: (1) ~50% of the total Al variation ( ~0.8 atoms p.f.u.) is due to the edenite exchange and (2) ~25-30% is due to a Tschermak-type Ti-Mn exchange. The P-sensitive Al-Tschermak substitution did not play a significant role. In order to constrain the ranges of absolute P and T over which these hornblendes crystallized and to assess the sensitivity of the recent thermo-barometric algorithms of Blundy and Holland (1990; BH90), Holland and Blundy (1994; HB94) and Anderson and Smith (1995; AS95), we have calculated P and T for two populations of analyses wherein Al₂O₃ contents are within analytical error (6.00+/-0.05 wt.% Al₂O₃, N=78 and 7.75+/-0.05 wt.% Al₂O₃, N=40). The barometric formulation of AS95 gives a mean P of 2.24+/-0.05 kb for the high-Al population at 760° C, which is indistinguishable from the 2.4+/-0.5 kb estimate of JR89. An excessive sensitivity to T at low P is suggested by the implausibly shallow depths calculated for the low-Al population (<1 kb at 760° C). The two thermometric formulations give reasonable results between 680° and 810° C, but the HB94 calibration gives a mean T higher by ~50° C and is overly sensitive to small analytical differences ( ~100° C spread for each population). Nine out of 14 traverses across large phenocrysts from different stratigraphic horizons of the Fish Canyon magmatic system display rimward increases in Al^T, (Na+K)^A, and Ti^M1-M3, compensated by decreases in Si^T, and Mn^M1-M3. Using the BH90 algorithm, the low-Al population, typical of near-core compositions, gives a mean T of 680-700° C, which is slightly above the water-saturated granite solidus at 2-2.5 kb. The high-Al population, typical of the rim composition, gives a value ~760-770° C, which is indistinguishable from independent T determinations using coexisting Fe-Ti oxides and Qtz-Mgt oxygen isotope thermometry. These profiles suggest that Fish Canyon hornblendes crystallized during near-isobaric reheating over a T range of ~80° C, which is consistent with the model of rejuvenation and remobilization of a pre-existing near-solidus crystal mush of batholithic dimensions via shallow intrusion of more mafic magma, as we have postulated for the Fish Canyon magma on the basis of independent textural/chemical evidence. Crystallization during reheating requires an open system, in which both heat and mass, in particular volatiles, are transferred from the underlying mafic magma. AS95: Am. Min. 80, 549-559. BH90: Contrib. Min. Pet. 104, 208-224. HB94: Contrib. Min. Pet. 116, 433-447. JR89: J. Pet., 30, 711-737.