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Ultra-high-pressure eclogites from the Dabie orogen that formed over a range in temperatures (600 to > 700°C) have been investigated with combined Lu-Hf and Sm-Nd geochronology. Three eclogites, sampled from Zhujiachong, Huangzhen and Shima, yield Lu-Hf ages of 240.0 ± 5.0, 224.4 ± 1.9 and 230.8 ± 5.0 Ma and corresponding Sm-Nd ages of 222.5 ± 5.0, 217.6 ± 6.1 and 224.2 ± 2.1 Ma respectively. Well-preserved prograde major-and trace-element zoning in garnet in the Zhujiachong eclogite suggests that the Lu-Hf age mostly reflects an early phase of garnet growth that continued over a time interval of c. 17.5 Myr. For the Huangzhen eclogite, despite preserved elemental growth zoning in garnet, textural study reveals that the Lu-Hf age is biased towards a later garnet growth episode rather than representing early growth. The narrow time interval of <6.6 Myr defined by the difference between Lu-Hf and Sm-Nd ages indicates a short final garnet growth episode and suggests a rapid cooling stage. By contrast, the rather flat element zoning in garnet in the Shima eclogite suggests that Lu-Hf and Sm-Nd ages for this sample have been reset by diffusion and are cooling ages. The new Lu-Hf ages point to an initiation of prograde metamorphism prior to c. 240 Ma for the Dabie orogen, while the exact peak metamorphic timing experienced by specific samples ranges between c. 230 to c. 220 Ma.

The Catalina Schist underlies the inner southern California borderland of southwestern North America. On Santa Catalina Island, amphibolite facies rocks that recrystallized and partially melted at ca. 115 Ma and 40 km depth occur atop an inverted metamorphic stack that juxtaposes progressively lower-grade, high-P/T rocks across low-angle faults. This inverted metamorphic sequence has been regarded as having formed within a newly initiated subduction zone. However, subduction initiation at ca. 115 Ma has been difficult to reconcile with regional geologic relationships since the Catalina Schist formed well after emplacement of the adjacent Peninsular Ranges batholith (PRB) had began in earnest. New detrital zircon U-Pb age results indicate that the Catalina Schist accreted over an ~20 m.y. interval. The amphibolite unit metasediments formed from Latest Neocomian to early Aptian (122 to 115 Ma) craton-enriched detritus derived mainly from the pre-Cretaceous wallrocks and Early Cretaceous volcanic cover of the PRB. In contrast, lawsonite-blueschist and lower grade rocks derived from Cenomanian sediments dominated by PRB plutonic and volcanic detritus were accreted between 97-95 Ma. Seismic data and geologic relationships indicate that the Catalina Schist structurally underlies the western margin of the northern PRB. We propose that construction of the Catalina Schist complex involved underthrusting of the Early Cretaceous forearc rocks to a subcrustal position beneath the western PRB. The heat for amphibolite facies metamorphism and anatexis observed within the Catalina Schist was supplied by the western PRB while subduction was continuous along the margin. Progressive subduction erosion ultimately juxtaposed the high-grade Catalina Schist with lower grade blueschists accreted above the subduction zone by 95 Ma. This coincided with an eastern relocation of arc magmatism and emplacement of the ca. 95 Ma La Posta tonalite-trondjhemite-granodiorite suite of the eastern PRB. Final assembly of the Catalina Schist marked the initial stage of the Late Cretaceous-early Tertiary craton-ward shift of arc 2 magmatism and deformation of southwestern North America that culminated in the Laramide orogeny.

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The 176Hf/177Hf ratio of seawater is measured directly with 143Nd/144Nd in a composite vertical profile in the NE Atlantic. The value of ɛHf of intermediate and deep water exhibits little variation and averages +1.0 ± 0.8; surface water shows a wider range, −5.7 to +3.3. While intermediate and deep water samples plot on, or close to, the seawater Hf‐Nd isotope array, defined by ferromanganese deposits, near‐surface seawater samples plot between the seawater and terrestrial rock ɛHf − ɛNd arrays. It is likely that the shallow water variability in ɛHf arises from a short residence time of Hf in surface water and isotopically heterogeneous inputs. The difference between surface and intermediate‐deep water ɛHf may be maintained by the release of surface scavenged Hf at depth from a subset of sinking particles; the composition of εNd at middepths reflects overlying surface water and water advected along isopycnals from areas where they outcrop in winter. Uniform ɛHf values below 1000 m suggest the residence time of Hf may be longer than that of Nd. The radiogenic isotopic composition of seawater Hf relative to Nd and terrestrial rocks suggests that minerals with high Lu/Hf ratios may be preferentially weathered, providing a source of radiogenic Hf to the oceans.

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Ultramafic blocks within mud-matrix mélange of the Franciscan Complex, California, preserve a series of metasomatic mineral zones generated by infiltration of Si-rich hydrous fluids during subduction. We describe the petrology and geochemistry of the metasomatic zones and compare them to current model predictions for the metasomatism of the mantle wedge by subduction zone fluids. Fluid flow affected a Cr-spinel lherzolite protolith to form first serpentinite, then a talc-dominated rock, and finally an amphibole-rich assemblage. A diverse suite of accessory minerals in the amphibole-rich zone (titanite ؉ clinozoisite ؉ zircon ؉ apatite) suggests that the trace element signature of subduction zone fluids may be fractionated in this zone. Oxygen isotopic evidence suggests that the ultramafic blocks equilibrated with metasomatic fluids during serpentinization and that subsequent reactions occurred in equilibrium with these fluids in a temperature range of 450-500 ؇C. Whole-rock geochemistry indicates mobility of many elements into and out of the blocks during metasomatism, including elements such as Ti which are currently considered to have low solubilities in such fluids.Taken as a whole, the blocks appear to preserve the metasomatic structure of the † Present address: slab-mantle interface in subduction zones and imply that the chemistry of slab-derived fluids is modified as they pass through these metasomatic zones in the mantle wedge. Our results suggest that the primary composition of subduction zone fluids is not likely reflected by arc magmas. Instead, we propose that arc magmas are derived from regions of the mantle fluxed by fluids residual to the metasomatic processes we observe.

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