GEOLOGIC FRAMEWORK OF THE NORTHERN PENINSULAR RANGES BATHOLITH, SOUTHERN CALIFORNIA

An asymmetrically zoned batholith formed over a period of 40 Ma as subduction magmatism transitioned from thin oceanic crust to thicker continental crust forming 3 zones, western(WZ), suture(SZ), and eastern(EZ). From 125 to 105 Ma WZ magma was passively emplaced in deformed low-grade Triassic and Jurassic metasedimentary rocks. Emplacement was largely by stoping, forming ring complexes and plutons with discordant contacts. Rocks range from olivine gabbro to granite, have isotropic fabric, contain equant-shaped mafic inclusions, and most have initial ⁸⁷Sr/⁸⁶Sr (Sr_i) 0.7034 to 0.7045.

From 105-100 Ma, subduction responding to a thick continental crust, changed from extensional to compressional, developing the SZ of complexly deformed prebatholithic rocks that include mantle rock. High-T continental metamorphic rocks were thrust over the thinner western crust producing Buchan-type metamorphic mineral assemblages. Pluton contacts, foliation, and inclusions are concordant with surrounding rocks. Rocks range from olivine gabbro to monzogranite; tonalite and granodiorite predominate. Most Sr_i is 0.7045 to 0.7055. Composite planar fabrics in the northernmost SZ rocks suggest “docking” with a terrane to the north.

EZ plutons are mostly large tonalitic plutons that young eastward from 99 to 83 Ma. Gabbro is rare. Country rock is Paleozoic to Proterozoic high-grade schist, marble and quartzite. Most batholithic rocks have less pronounced planar fabrics and less attenuated mafic inclusions than in the SZ. Most tonalitic Sr_i is 0.7065 to 0.7080 and gabbro as low as 0.7031. Sphene, sparse in WZ and SZ, is an abundant accessory mineral.

Si, K, Li, Zn, Rb, Sr, Nb, Ba, La, Ce, Nd, and Ta increase WZ to EZ and Mg, Fe, Ca, Sc, V, and Yb decrease. Sr_i is bimodal with few rocks of Sr_i 0.7055 – 0.7065. ¹⁸O averages 6.8 WZ, 9.7 SZ, and 9.8 EZ. Pb 207/204 average 15.596 WZ, 15.614 SZ, and 15.683 EZ; Pb 206/204 average 38.404 WZ, 38.573 SZ, and 38.803 EZ. Batholith pressures increase from 1.6 - 3.5 kbar WZ, 5 - 6 SZ, decrease to 4 - 5 EZ and increase to 6 - 6.5 in the easternmost EZ. Magnetic susceptibility values are high in the west and low in the east; some plutons in the suture zone are magnetically zoned. Average specific gravity decreases from 2.77 WZ to 2.73 EZ.