Crustal and magmatic evolution in a large multicyclic caldera complex; isotopic evidence from the central San Juan volcanic field.

Abstract

The Sr, Nd and Pb isotope compositions of ash-flow tuffs and lavas from the central caldera cluster of the San Juan volcanic field, Colorado, suggest that the silicic magmas were derived by fractional crystallization of mantle-derived basalts, coupled with extensive assimilation of both lower-and upper-crustal components. Temporal trends of increasing epsilon sub Nd values and decreasing super 87 Sr/ super 86 Sr ratios of the ash-flow tuffs suggest that extensive crustal hybridization of both upper-and lower-crustal reservoirs occurred as a result of magmatism. Mantle-derived basalts are envisioned to have initially crystallized and significantly interacted with crust near the crust-mantle boundary, creating a hybrid crust that is a mixture of mantle and lower-crustal components. Evolved magmas ascended into the upper crust, where they continued to assimilate and crystallize, modifying the bulk upper-crustal composition through transfer of both lower-crustal and mantle components into the upper crust, strongly affecting the isotopic compositions of the lower and upper crust. Lower-crustal epsilon sub Nd values and super 87 Sr/ super 86 Sr ratios are calculated to have shifted >30-40% toward mantle compositions during these processes, although Pb isotope compositions of the lower crust are not as strongly affected <30% shift to mantle compositions. Depending on the extent of upper crust-lower crust recycling, upper-crustal epsilon sub Nd values shift 15-30% toward mantle compositions, super 87 Sr/ super 86 Sr ratios shift 20-55% toward lower-crustal and mantle ratios and super 206 Pb/ super 204 Pb 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 localized to the region immediately underlying the calderas. Two distinct 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 relatively unhybridized, original Proterozoic crust. Isotopic compositions of the caldera-related lavas are variable, and unlike the ash-flow tuffs, there are no distinctive temporal trends. The lavas are believed to undergo late-stage evolution in smaller magma chambers surrounding the caldera cluster, where the crust has not been as intensely hybridized. The comparatively large isotopic variability of the lavas reflects the high susceptibility of small magma chambers to isotopic modification 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 isotopic compositions of the tuffs serve as a better monitor of changes in the average composition of the magmatic system and the crustal column.

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