San Jacinto intrusive complex: 2. Geochemistry

Rocks from three large (>100² km)tonalitic intrusions exposed in the San Jacinto Mountains of southernCalifornia show a restricted compositional range of between 63 and 68 wt % SiO₂ forall but volumetrically minor felsic differentiates (with Si0₂≈70 wt%). All rocks with less than 65.5 wt % SiO₂ show linearelement-element covariation. Felsic differentiates have characteristics (higherSiO₂, K₂O, Rb, Ba, U; higher and variable rare earthelements) consistent with derivation by in situ fractionation; rocks withbetween 65.5 and 70 wt % SiO₂ have intermediate characteristicsand are interpreted as derived from liquids formed by mixing “primitive”liquids with fractionated liquids within an intermittently recharged,continuously solidifying magma chamber. Mafic inclusions extend thecompositional trends of the mafic tonalites to 55 wt % SiO₂. Thechemical variations of both inclusions and more mafic tonalites are interpretedas resulting from processes acting before injection of their parental liquidsinto the observed crustal magma chambers. Effects of chamber processes areminor for all but the most felsic rocks. The major effect of recharge is tobuffer the thermal and chemical properties of liquids within the magmachambers, yielding large volumes of relatively homogeneous tonalite. For thoseelements where the bulk distribution coefficient is between about 0.5 and 2,concurrent recharge and solidification produces rocks that closely approximatethe composition of the added liquids. Estimated Rayleigh numbers for theseliquids are high (>10¹⁰), implying convection throughout much ofthe solidification history of each chamber. Existence of trace elementvariations within analyzed rocks imply that convection was not totallyefficient at homogenizing the various batches of liquid added to each chamber.