Distinctive Triassic megaporphyritic monzogranite: Evidence for only 160 km offset along the San Andreas Fault, southern California

Abstract

Distinctive megaporphyritic bodies of monzogranite to quartz monzonite that occur in the Mill Creek region of the San Bernardino Mountains and across the San Andreas fault on Liebre Mountain share identical modal and chemical compositions, intrusive ages, and petrogenesis and similar thermal histories. Both bodies are strontium‐rich and contain large potassium feldspar phenocrysts and hornblende. U‐Pb determinations on zircon from both bodies indicate Triassic intrusive ages (215 Ma) and derivation, in part, from homogeneous Precambrian continental crust. U‐Pb analyses on apatite and sphene and K‐Ar analyses on hornblende and biotite show that the bodies suffered a Late Cretaceous thermal event (70–75 Ma). The strong similarities between the two bodies suggest that they constitute segments of a formerly continuous pluton that has been offset about 160 km by movement on the San Andreas fault, about 80 km less than the generally accepted distance. Plutons having monzonitic compositions, reassembled with the megaporphyritic bodies are used as a piercing point, form a relatively coherent province within the varied suite of Mesozoic batholithic and prebatholithic rocks in southern California.