A RECONNAISSANCE STUDY OF GRANITE SUITES ACROSS SAN DIEGO COUNTY USING 176Hf/177Hf FROM SINGLE ZIRCON GRAINS: EVIDENCE OF MAGMA MIXING BETWEEN MANTLE-DERIVED AND CRUSTAL-DERIVED MELTS

Magma mixing was important in the genesis of granites from the Peninsular Ranges batholith, San Diego County. Depleted mantle ages (Hf T_DM) calculated from zircon ¹⁷⁶Hf/¹⁷⁷Hf ratios for 16 plutons indicate a significant crustal component in granites from both western and eastern zones. Fundamental differences in crustal structure are indicated –a Neoproterozoic crustal component in western-zone granites and a more complex, possibly layered crust of Neoproterozoic and older aged components to the east.

Zircon U-Pb age determinations for four western-zone I-type granite suites and one gabbro suite yield Early Cretaceous emplacement ages between ~114 Ma and 101 Ma. Combined zircon ε_Hf values for these five suites vary between +2.5 and +11.2 representing depleted mantle model ages between ~650 Ma and ~300 Ma. As the older model-aged zircons may have crystallized from hybrid magma rather than from a crustal-derived melt, the calculated age for the crustal end-member is considered a minimum. Given that the samples have low initial ⁸⁷Sr/⁸⁶Sr and low to moderate δ¹⁸O, the range of T_DM ages for five I-type western suites suggests a dominant Neoproterozoic, low LILE partial melt of mafic crustal material mixed with a Cretaceous mantle-derived fractionated melt component.

Zircon U-Pb data from the eastern zone verify the presence of Jurassic I- and S-type plutons (~160 Ma) and Early to Late Cretaceous I-type plutons (~109 to 94 Ma). For a Jurassic I-S-type granite, zircon ε_Hf values vary between -2.5 and +3.4 representing T_DM model ages between ~900 Ma and ~650 Ma. For three Cretaceous suites of the eastern zone, zircon ε_Hf values vary between -21.2 and +12.2 representing depleted mantle model ages between ~1,550 Ma and ~280 Ma. The range of zircon T_DM ages indicates several crustal source components: 1) a lower crustal Mesoproterozoic component, in part eclogitic, for the Late Cretaceous high Sr/Y La Posta granite; 2) a Neoproterozoic crustal component for Jurassic I-S-type plutons; and 3) a Mesozoic metasedimentary component derived from reworked Proterozoic crust for Jurassic S-type plutons.