Rare Earths Abundances and Fractionations and their Implications for Batholithic Petrogenesis in the Peninsular Ranges Batholith, California, USA, and Baja California, Mexico

Abstract

Rare Earth Element (REE) patterns of plutonic rocks across the Peninsular Ranges batholith vary systematically west to east, transverse to the long axis and structural trends of the batholith. Three major parallel elongate geographic regions are each defined by distinct REE pattern types. Rocks from the western region display slight light REE enrichment, flat heavy REE, and negative Eu anomalies. An abrupt transition to rocks with middle and heavy REE fractionated and depleted REE patterns with no or positive Eu anomalies occurs in the central region of the batholith. Further to the east a second transition to strongly light REE enriched rocks some of which have positive or negative Eu anomalies occurs. Some gabbros may show divergent patterns. These large variations are observed even in similar lithologies across the three regions and notably in tonalites, the major rock type of the batholith. The slopes of the REE patterns within rocks of each region are largely independent of rock type, and no consistent variations in REE abundances and Eu anomalies with lithology are noted with the exception of some gabbros. Most of the leucogranodiorites of the western region have larger negative Eu anomalies than nearby tonalites. Granodioritic rocks of the central and eastern regions may have positive, negative, or no Eu anomalies. These results are the first report of systematic variations in REE characteristics across a granitic batholith whose geologic setting at a convergent plate boundary has been established. Some similarities and contrasts to REE variations across modern volcanic arcs are noted. Along the westernmost margin of the batholith in northern Baja California, Mexico, leucotonalitic rocks of the San Telmo pluton display essentially flat REE patterns strongly resembling those observed for near-trench volcanic rocks. The REE patterns of quartz gabbros and tonalites of the western region correspond closely to those of circum-Pacific high-alumina basalts. The heavy REE depleted and fractionated patterns observed in the rocks of the central and eastern regions of the batholith do not have counterparts in oceanic island volcanic arcs, and few counterparts in continental margin volcanic arcs. The REE variations generally correlate with other transverse asymmetries in major petrologic and geochemical characteristics. The abrupt depletion and fractionation in the middle to heavy REE and elimination of negative Eu anomalies appear coupled to an increase in Sr concentration and a marked restriction in lithologic diversity. This transition occurs over a range of initial 87Sr/86Sr ratios. The light REE enriched rocks of the eastern region are distinguished from the central and western regions by higher initial ratios. Geographic discontinuities in δ18O and age distributions in the batholith correlate approximately with the REE discontinuities, but locally diverge by the dimensions of one or two plutons. Determinations of REE abundances in major and trace phases of a representative eastern region granodiorite indicate accessory sphene and allanite are the major reservoirs of REE in this rock. Hornblende is the only significant REE site in the major minerals, and in some batholithic lithologies it may be the dominant site. High-level crystal fractionations involving hornblende and accessory phases do not appear capable of producing the observed geographic characteristics. Contamination processes including upper crustal material also seem ruled out. The REE and other geochemical variations across the batholith appear to originate in deep-seated sources. Partial melting in source rocks in which assemblages rich in plagioclase give way laterally to garnet-bearing assemblages in source regions of broadly basaltic composition which are already zoned in light REE abundances, 87Sr/86Sr, δ18O, and possibly Sr content appears to account for most of the observed features. The geologic context of the source material remains largely undefined and may include mantle and crustal components. However, the source regions for all parts of the batholith must have bulk compositions and phase assemblages capable of producing the dominant tonalite and low-K2O granodiorite lithologies. This major constraint appears to strongly limit the amount of sialic crustal material permitted to be present in the source regions. The geometry of the convergent boundary appears to have determined the elongate form of the batholith, and, probably, the general alignment of all the geochemical variations along its length. The results of this study may be useful in comparing possibly related crust-forming processes and products in other orogenic-plutonic terrains.