SYNDEFORMATIONAL MELTING IN MESOZOIC TONALITES AND METAPELITES FROM THE SIERRA SAN PEDRO MARTIR AREA, BAJA CALIFORNIA, MEXICO

The timing of regional deformation and migmatization of wallrocks to the multi-aged and multi-component Peninsular Ranges Batholith (PRB) in northern Baja California is unresolved and controversial. In the Sierra San Pedro Martir area extensive migmatites occur near plutons ranging in age from Middle Jurassic to Early Cretaceous. Our work, in part, compares the relative and absolute timing between migmatite development, deformation, and pulses of magmatism to better understand the thermo-mechanical evolution of the batholith.

Recent mapping has focussed on a Mesozoic amphibolite grade package of flysch east of the Main Martir thrust, which includes sparse but locally continuous metapelitic schists and gneisses, some showing graded bedding. East-dipping, overturned schists and gneisses locally contain late- or post-kinematic 1-2 cm garnet, as well as randomly oriented sillimanite after andalusite. These rocks are locally folded with fold axes parallel to a crenulation that increases in intensity to the east, and with proximity to ca. 135 Ma deformed tonalites. Meltbands within these tonalites are concordant to the regional foliation and in places localized in conjugate shearbands. Melt domains in older para- and orthogneisses are localized in fold hinges, shearbands and low strain zones. These observations suggest syndeformational melting. The foliation-concordant meltbands in ca. 135 Ma tonalites are locally folded near younger tonalites that range in age from ca. 108 – 103 Ma.

Our work shows that migmatization occurred syndeformationally between ca. 135 Ma and 108 Ma, possibly in association with an intervening magmatic event at ca. 120 Ma, documented elsewhere in the Sierra San Pedro Martir. Further work may show that migmatization also occurred prior to 135 Ma, in association with an extensive Middle Jurassic magmatic event, evidence for which is now preserved as widespread orthogneiss.