Mesozoic structural domes are developed in an older Proterozoic crystalline basement of granitic to granodioritic foliated metaplutons in the Halloran Hills. Isotopic analyses of whole- rocks and mineral separates from these rocks by U/Th/Pb, Rb/Sr, and Ar/Ar techniques yield a complex discordance. Such discordance resulted from metamorphism of approx 1700 m.y. plutonic rocks during the Jurassic (= or > 140-150 m.y.) and subsequent uplift and cooling to approx 200oC at about 90 m.y. On the basis of this study, the isotopic dating systems ranked in decreasing order of resistance to resetting are: U/Th/Pb zircon (concordia intercept) = or > Rb/Sr whole-rock approx Rb/Sr amphibole approx U/Th/Pb amphibole approx Pb/Pb whole-rock > Ar/Ar amphibole = or > Rb/Sr sphene = or > U/Pb sphene and apatite > Rb/Sr plagioclase-K feldspar-apatite > Th/Pb sphene and apatite approx Ar/Ar biotite approx U/Pb feldspars > Rb/Sr biotite.-L.-diH.
Additional publication details
U-Th-Pb, Rb-Sr, and Ar-Ar mineral and whole-rock isotopic systematics in a metamorphosed granitic terrane, southeastern California.