The collision of the Sino-Korean and Yangtze blocks to form a significant part of China is recorded in the Qinling, Tongbai, and Dabie Mountains. Radiometric ages of the ultrahighpressure metamorphic rocks in the South Qinling orogenic belt suggest that subduction and collision took place during the Triassic Period. Our new 40 Ar/ 39 Ar geochronology of units in the North Qinling orogenic belt confirms that high-grade metamorphism and deformation took place also during the Silurian-Devonian and Carboniferous Periods. These results imply that the amalgamation of eastern China was a multistage process extending over at least 200 m.y. GEOLOGY, April 1998Figure 4. Disturbed spectra and inverse correlation diagrams. Abbreviations: MSWD-mean squared weighted deviations; others as in Figure 3. A: Locality 3. Saddle-shaped hornblende spectrum from Erlangping amphibolite. B: Locality 3. Inverse correlation diagram yielding isochron age of 404 Ma; the trapped component has 40 Ar/ 36 Ar = 1019, significantly higher than the atmospheric value of 295.5. C: Locality 5. Disturbed spectrum yields only a total fusion age of 365 ± 2 Ma. D: Locality 5. Inverse correlation diagram defines isochron age of 304 Ma with 40 Ar/ 36 Ar ratio of 5026.

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These data support the hypothesis that residential proximity to NOA is significantly associated with increased risk of MM in California.

The Franciscan subduction complex of California is considered a type example of a subduction-accretion system, yet the age of subduction initiation and relationship to the tectonic history of western North America remain controversial. Estimates for the timing of Franciscan subduction initiation are largely based either indirectly on regional tectonic arguments or from the ages of high-grade blocks within mélange. Many of the high-grade blocks record counterclockwise pressure-temperature paths with early amphibolite overprinted by later eclogite and blueschist; however, their origin and significance with respect to subduction initiation have been debated. In contrast, some high-grade blocks show evidence for clockwise pressure-temperature paths and an early eclogite assemblage overprinted by later amphibolite. Zircon U-Pb ages from inclusions in garnet and Lu-Hf estimates of initial garnet growth ages from these samples record early eclogite metamorphism at~176 Ma. Matrix zircon U-Pb ages and Lu-Hf estimates of final garnet growth ages record a barroisite-amphibolite assemblage overprint of eclogite at~160 Ma. Combined with petrologic data and existing geochronology, the data suggest that (1) Franciscan subduction was underway by no later than 180 Ma, (2) continuous subduction metamorphism occurred for at least 100 Ma, and (3) Franciscan subduction initiation predated the formation of the overlying Coast Range Ophiolite, supporting models that form the ophiolite above an east dipping Franciscan subduction zone.

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Our understanding of low-grade, mafic, metamorphic rocks is relatively primitive compared to higher grade equivalents, in large part because of the abundance of relict minerals, the difficulties of studying fine-grained rocks, and the difficulties of establishing the existence and scale of chemical equilibrium. However, work carried out during the past decade has identified the systematic correspondence between effective bulk compositions and mineral assemblage that is required by the metamorphic facies concept, and a large body of data defines the mineral pangeneses of coexisting pumpellyite (Pmp), prehnite (Prh), amphibole (Ab), and epidote (Ep). At pressures and temperatures typical of many regional metamorphic ter- rains, rocks containing Ep + Ab + quartz (Qtz) may have, in addition, chlorite (Chi) + actinolite (Act) in magnesian bulk compositions, Chi + Pmp in more ferroan compositions, and Prh in compositions relatively lower in aluminum and ferric iron. Act + Pmp ± Chi ± Prh may coexist in a narrow range of compositions with intermediate values of Mg/(Mg + Fe 2+ ). Improvements in thermodynamics data bases now permit calculation of pressures and temperatures for low variance assemblages.The results of systematic examination of mineral assemblages and calculations of petrogenetic grids for epidote-bearing rocks suggest that the stability field of the Prh-Pmp facies is entirely contained within the overlapping stability fields of the Pmp-Act and Prh-Act facies. At pressures typical of the most common metamorphic field gradients, the stability fields of the three facies cannot be distinguished.The predictability of mineral assemblages and the recent success in calculating meaningful pressure and temperature for low-grade rocks suggest that, under favorable circumstances, mineralogical approaches rooted in the assumptions of equilibrium thermodynamics are likely to prove as useful for determining the physical conditions of low-grade metamorphism as they have been for high-grade rocks.