CRUSTAL AND MAGMATIC EVOLUTION IN A LARGE MULTICYCLIC CALDERA COMPLEX- ISOTOPIC EVIDENCE FROM THE CENTRAL SAN-JUAN VOLCANIC FIELD

The Sr, Nd and Pb isotope compositions of ash-flow tuffs and lavas fro m the central caldera cluster of the San Juan volcanic field, Colorado , suggest that the silicic magmas were derived by fractional crystalli zation of mantle-derived basalts, coupled with extensive assimilation of both lower-and upper-crustal components, Temporal trends of increas ing epsilon(Nd) values and decreasing Sr-87/Sr-86 ratios of the ash-fl ow tuffs suggest that extensive crustal hybridization of both upper-an d lower-crustal reservoirs occurred as a result of magmatism. Mantle-d erived basalts are envisioned to have initially crystallized and signi ficantly interacted with crust near the crust-mantle boundary, creatin g a hybrid crust that is a mixture of mantle and lower-crustal compone nts. Evolved magmas ascended into the upper crust, where they continue d to assimilate and crystallize, modifying the bulk upper-crustal comp osition through transfer of both lower-crustal and mantle components i nto the upper crust, strongly affecting the isotopic compositions of t he lower and upper crust, Lower-crustal epsilon(Nd) values and Sr-87/S r-86 ratios are calculated to have shifted > 30-40% toward mantle comp ositions during these processes, although Pb isotope compositions of t he lower crust are not as strongly affected (< 30% shift to mantle com positions). Depending on the extent of upper crust-lower crust recycli ng, upper-crustal epsilon(Nd) values shift 15-30% toward mantle compos itions, Sr-87/Sr-86 ratios shift 20-55% toward lower-crustal and mantl e ratios and Pb-206/Pb-204 ratios decrease 15-25% toward lower-crustal ratios. Crustal hybridization during evolution of the central caldera cluster was intense, although hybridization in the upper crust was lo calized to the region immediately underlying the calderas. Two distinc t isotopic cycles are recognized in the ash-flow tuffs, one from 27.8 to 26.9 Ma (first caldera cycle) and the second from 26.4 to 26.1 Ma ( San Luis caldera cycle),The break between the two cycles may be due to a shift in the locus of magmatism to the northwest, into an area of r elatively unhybridized, original Proterozoic crust, Isotopic compositi ons of the caldera-related lavas are variable, and unlike the ash-flow tuffs, there are no distinctive temporal trends. The lavas are believ ed to undergo late-stage evolution in smaller magma chambers surroundi ng the caldera cluster, where the crust has not been as intensely hybr idized. The comparatively large isotopic variability of the lavas refl ects the high susceptibility of small magma chambers to isotopic modif ication during interaction with heterogeneous crust, In contrast, ash- flow magmas evolve in much larger magma chambers which tend to average isotopic heterogeneity of crust and mantle end-members, and the isoto pic compositions of the tuffs serve as a better monitor of changes in the average composition of the magmatic system and the crustal column.