CHEMICAL STRATIFICATION OF THE CRUST: ISOTOPE, TRACE ELEMENT, AND MAJOR ELEMENT CONSTRAINTS FROM CRUSTALLY CONTAMINATED LAVAS AND LOWER CRUSTAL XENOLITHS by MARY

The dehydration of the lower crust and the assimilation of crustal materials into ascending magmas are two geologic phenomenon which lead to vertical mass transport and potentially, geochemical stratification of continental crust. LOWER CRUSTAL XENOLITHS, KILBOURNE HOLE This integrated Sr-Nd-Pb isotope study of paragneiss xenoliths from Kilbourne Hole maar, New Mexico, is the first of its kind applied to materials derived directly from the lower crust (6-10 kb). The pelitic paragneisses represent shales incorporated into the lower crust, in themselves exemplifying the importance of tectonic and sedimentary processes to the evolution of the lower crust. Because shales are rich in elements conventionally thought to be depleted in the lower crust, the pelitic paragneisses are an important geochemical reservoir to consider in the lower crust; their response to high grade metamorphism is pertinent to the problem of intracrustal stratification. Sr isochrons determined from layers at a xenolith (decimeter) scale yield -1.6 Ga ages, coeval with metamorphic ages in basement rocks exposed at the surface. Locally, minerals are in isotopic equilibrium as a result of recent reheating of the lower crust related to Rio Grande rifting. Pb isochrons yield discordant ages as a result of this heating but are internally consistent with tectonothermal stability of the crust throughout the remainder of the Precambrian and most of the Paleozoic in spite of proximity to anorogenic plutonism at -1.4 Ga and orogenic activity at -1.0 Ga. Nd isochrons also yield discordant ages suggesting the heretofore unexpected possibility that effective diffusion rates for Nd are faster than Sr under dry crustal conditions. The paragneisses exhibit a wide range in internal composition. A representative Sm/Nd ratio for the paragneisses as a whole is obtained by considerations based on their time-integrated Nd isotope evolution. Other geochemical characteristics of the paragneisses are be correlated to Sm/Nd and a representative major and trace element composition for the paragneisses is determined. Although similar in many respects to typical shales, the paragneisses are significantly depleted in U and Cs and may have been depleted by a factor of -2 in Th and the LREE relative to the protolith during metamorphism. High LREE abundances measured in the feldspars are unlikely to be maintained in the presence of a melt. Consequently, the paragneisses were probably depleted by means other than partial melting. Relative to average crust, the pelitic paragneisses are radiogenic in 87Sr/ 8 Sr, are unradiogenic in 2 0 6 Pb/ 20 4 Pb and 2 0 7 Pb/ 2 0 Pb, and have 14 Nd/ 1 4 Nd similar to middle Proterozoic crust. Existing data also show that they retain the relatively high 6180 of their precursors. In contrast, isotopic data for orthogneisses from Kilbourne Hole indicate that they are unradiogenic in 7Sr/ 8 Sr and may be more variable, albeit largely unradiogenic, in 1 Nd/14 Nd. Consequently, representative isotope characteristics for the lower crust as a whole depend on the relative proportion of these different lithologies. If pelitic paragneisses are present at geologically reasonable 10-20% abundance levels in the lower crust, they reduce the heat flux required of the subcontinental mantle by up to 50%. They increase 8 Sr/ 8 Sr by up to 0.006 and incompatible elements abundances by 20-100% over previous estimates for the lower crust. On the other hand, the unradiogenic Pb isotopic signature of the lower crust, previously suspected from studies of granulites exposed at the surface, is confirmed. CRUSTAL ASSIMILATION, KODIAK ISLAND, ALASKA, AND PARICUTIN VOLCANO, MEXICO Crustal assimilation in two disparate tectonic settings is examined in order to understand its nature and timing. The Ghost Rocks volcanics (GRV) of Kodiak Island, Alaska are early Tertiary submarine basalts and andesites which erupted in a near-trench setting. Subduction of the Kula-Farallon spreading center beneath the Kodiak accretionary prism is proposed as the ultimate source of the GRV on the bases of tectonic considerations and MORB-like isotopic and trace element signatures in the basalts. Andesites which are conformably overlain by the basalts have less radiogenic Nd isotope signatures and arc-like chemical affinities. Inversion of selected trace element characteristics in light of the Nd isotope data shows them to be the product of mixing between the basalts and sediments of the accretionary prism. The lavas of Paricutin volcano, a cinder cone in Mexico, exhibit linear trends in isotopic and incompatible element ratios consistent with crustal assimilation. However, they exhibit a tripartite trend in elemental abundances over time and more mafic lavas are richer in many of the incompatible elements than those which have undergone greater amounts of crustal assimilation. This is suggested to reflect the diachronous interplay of crystal fractionation with crustal