Mineral Chemistry of the Tuolumne Intrusive Suite: Evidence for Disequilibrium and Implications for Estimated Magmatic Intensive Variables

The Tuolumne Intrusive Suite (TIS) is one of a number of zoned plutonic bodies emplaced along the eastern Sierra Nevada batholith (SNB) during the Late Cretaceous. The compositions of the rocks range from marginal tonalities to granodiorites to granite toward the interior. Similar to most other plutons of the SNB, the rocks are metaluminous, high-K, calcalkaline graniotoids with mineralogy consisting predominantly of K-feldspar, plagioclase, biotite, hornblende, and quartz, and accessory magnetite, titanite, apatite, and zircon. Evidence of alteration is evident by the localized presence of secondary epidote, sericite, and chlorite. Microprobe analysis of unaltered feldspars reveals that compositions do not agree with those determined from equilibrium experiments; the K-feldspars are generally devoid of the An component and the plagioclases lack the Or component. This is inconsistent with crystallization compositions and suggests subsolidus alteration. Unaltered biotites and hornblendes are uniformly Mg rich suggesting high fO2 during emplacement. Fe/(Fe+Mg) ratios vary from 0.41 to 0.44 and 0.31 to 0.43, respectively, decreasing with whole rock wt.% SiO2 (toward the pluton interior). Biotite compositions reveal that fO2 varied from 10-11 to 10-13, a value moderately higher than has been previously suggested from titanite+magnetite+quartz equilibrium relationships. Analysis of Mg-Fe exchange between hornblende and biotite reveals KD values that range from 1.23 to 1.64, demonstrating that hornblende is the predominant reservoir for Mg in these rocks. In contrast with previous experimental and theoretical studies, KD values are poorly correlated with hornblende compositions. There is no strong relationship between KD and total Al or A-site occupancy. Relationships between Mg/(Fe+Mg) and KD are also generally absent. Lack of strong relationships between these variables is inconsistent with crystallization of biotite and hornblende under equilibrium conditions, further suggesting subsolidus alteration. Disequilibrium has consequences for estimation of magmatic intensive variables. A high Mg content (KD >>1) will cause hornblende to acquire less total Al when compared to hornblende-biotite equilibrium conditions. Thus any barometric estimates based on equilibrium Al-in-hornblende relationships will result in unrealistically low pressure values. In addition, use of feldspar thermometry will result in incorrect crystallization temperatures due to loss of alkalies from the feldspar phases.