A hypothesis for a late Precambrian fit of western North America with the Australia-Antarctic shield region permits the extension of many features through Antarctica and into other parts of Gondwana. Specifically, the Grenville orogen may extend around the coast of East Antarctica into India and Australia. The Wopmay orogen of northwest Canada may extend through eastern Australia into Antarctica and thence beneath the ice to connect with the Yavapai-Mazatzal orogens of the southwestern United States. The ophiolitic belt of the latter may extend into East Antarctica. Counterparts of the Precambrian-Paleozoic sedimentary rocks along the U.S. Cordilleran miogeocline may be present in the Transantarctic Mountains. Orogenic belt boundaries provide useful piercing points for Precambrian continental reconstructions. The model implies that Gondwana and Laurentia rifted away from each other on one margin and collided some 300 m.y. later on their opposite margins to form the Appalachians.

Southwest U.S.-East Antarctic (SWEAT) connection: A hypothesis

Phanerozoic tectonics of the South China Block: Key observations and controversies

Principles of Isotope Geology

The prospect of partial melting of the subducted oceanic crust to produce arc magmatism has been debated for over 30 years. Debate has centred on the physical conditions of slab melting and the lack of a definitive, unambiguous geochemical signature and petrogenetic process. Experimental partial melting data for basalt over a wide range of pressures (1–32 kbar) and temperatures (700–1150°C) have shown that melt compositions are primarily trondhjemite–tonalite–dacite (TTD). High-Al (> 15% Al 2 O 3 at the 70% SiO 2 level) TTD melts are produced by high-pressure (≥ 5 kbar) partial melting of basalt, leaving a restite assemblage of garnet + clinopyroxene ± hornblende. A specific Cenozoic high-Al TTD (adakite) contains lower Y, Yb and Sc and higher Sr, Sr/Y, La/Yb and.Zr/Sm relative to other TTD types and is interpreted to represent a slab melt under garnet amphibolite to eclogite conditions. High-Al TTD with an adakite-like geochemical character is prevalent in the Archean as the result of a higher geotherm that facilitated slab melting. Cenozoic adakite localities are commonly associated with the subduction of young ( −1 ) conducive for slab dehydration melting. Viable alternative or supporting tectonic effects that may enhance slab melting include highly oblique convergence and resultant high shear stresses and incipient subduction into a pristine hot mantle wedge. The minimum P–T conditions for slab melting are interpreted to be 22–26 kbar (75–85 km depth) and 750–800°C. This P–T regime is framed by the hornblende dehydration, 10°C/km, and wet basalt melting curves and coincides with numerous potential slab dehydration reactions, such as tremolite, biotite + quartz, serpentine, talc, Mg-chloritoid, paragonite, clinohumite and talc + phengite. Involvement of overthickened (>50 km) lower continental crust either via direct partial melting or as a contaminant in typical mantle wedge-derived arc magmas has been presented as an alternative to slab melting. However, the intermediate to felsic volcanic and plutonic rocks that involve the lower crust are more highly potassic, enriched in large ion lithophile elements and elevated in Sr isotopic values relative to Cenozoic adakites. Slab-derived adakites, on the other hand, ascend into and react with the mantle wedge and become progressively enriched in MgO, Cr and Ni while retaining their slab melt geochemical signature. Our studies in northern Kamchatka, Russia provide an excellent case example for adakite-mantle interaction and a rare glimpse of trapped slab melt veinlets in Na-metasomatised mantle xenoliths.

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Petrogenesis of slab-derived trondhjemite–tonalite-dacite/adakite magmas

The chemical compositions of primary magmas of olivine tholeiite (OTB), high-alumina basalt (HAB), and alkali olivine basalt (AOB) are obtained by the olivine maximum fractionation model for Quaternary magnesian basalts from the Northeastern Japan arc. These basalts are assumed to have fractionated only olivine crystals before eruption. The melting phase relations for three primary basalt compositions have been determined under both anhydrous and water-undersaturated conditions. The AOB melt coexists with olivine, orthopyroxene, and clinopyroxene at 17 kbar and 1360°C under anhydrous conditions and at 23kbar and 1320°C in the presence of 3wt % water. The HAB melt also coexists with the above three phases at 15 kbar and 1340°C under anhydrous conditions and at 17 kbar and 1325°C in the presence of 1.5 wt % water. The OTB melt, on the other hand, coexists with olivine and orthopyroxene at 11 kbar and 1320°C under anhydrous conditions. The water contents in arc basalt magmas are estimated to be about 3, 1.5, and nearly O wt % for the AOB, HAB, and OTB, respectively, on the basis of the solubility limit of water in silicate melts. Based on these estimates and the experimental results, the AOB, HAB, and OTB magmas are suggested to segregate from the mantle at about 1320°C and at 23, 17, and 11 kbar, respectively. As the temperatures at the segregation of the magmas given above appear to be too high for a stable mantle geotherm, the mantle diapir is the most probable mechanism for the magma production in a subduction zone. Considering the heat of formation of melt in the diapir, the region with temperatures higher than 1400°C has to be present in the mantle wedge.

Generation of arc basalt magmas and thermal structure of the mantle wedge in subduction zones

Late Cretaceous (90–100 Ma) A-type granites are widespread in the coastal area of the Zhejiang and Fujian Provinces, SE China. According to mineralogical and geochemical characteristics, the A-type granites in this belt can be further divided into aluminous and peralkaline subgroups. The aluminous subgroup often contains aluminous-rich minerals (e.g. spessartine and Mn-rich muscovite), while the peralkaline subgroup usually contains riebeckite, arfvedsonite and aegirine. Geochemically, the aluminous A-type granites show lower Nb, Zr, Ga, Y and REE abundances, and lower FeO*/MgO and Ga/Al than the peralkaline subgroup. When they occur in the same area, the two subgroups of A-type granites display quite similar initial Nd isotopic compositions, which are indicative of mixing of ancient basement crustal rocks with variable amounts of mantle materials. Integrated geological and geochemical investigations indicate that both the aluminous and the peralkaline magmas are highly evolved and reflect the residual liquids left after high degrees of fractional crystallisation in a deep magma chamber. The present authors suggest that the mineralogical and geochemical differences between the aluminous and peralkaline subgroups are likely to have been generated via different differentiation paths controlled by varying fluorine contents of the parent magmas.

Two subgroups of A-type granites in the coastal area of Zhejiang and Fujian Provinces, SE China: age and geochemical constraints on their petrogenesis

Early Cretaceous plutonic rocks in the Kitakami Mountains, northeast Japan, related to the westward subduction along the eastern margin of the East Asian continent, intruded the Jurassic accretionary complex and a microcontinental block. These plutonic rocks are classified into types A and B. The former is further subdivided into high-Sr, low-Sr, and intermediate series. High-Sr series rocks occur as the central parts of three zoned plutons in the North Kitakami belt; named the Hashigami, Tanohata, and Miyako plutons. The high-Sr series rocks show distinct chemical characteristics compared with those of the low-Sr series. In particular, the high-Sr series is characterized by SiO2 greater than 61 wt %, higher Na2O and lower K2O concentrations, and slightly higher Al2O3 and lower total FeO and MgO concentrations. In trace element chemistry, the high-Sr series rocks are richer in Sr and Ga, and poorer in Rb, Pb, and Y. These characteristics resemble those of the high-Al2O3 trondhjemite, tonalite, and dacite (high-Al TTD). Mass balance calculation indicates that the high-Sr series magma can be derived by partial melting of hypothetical oceanic crust (mid-oceanic ridge basalt (MORB), plus 5% altered MORB, plus 5% sediments) leaving garnet, clinopyroxene, quartz, rutile, and apatite as restite. On the basis of the phase relations for tholeiitic basalt and the estimated geothermal gradient of the subducted slab, slab melting is considered to have occurred at a depth of 2.0 to 2.2 GPa (about 70 to 80 km), and the age of subducted oceanic crust may be younger than 10 to 25 Ma. The coexistence of the magma derived from the subducted slab with that derived from the mantle wedge is a particularly important characteristic of the Early Cretaceous magmatism in the Kitakami Mountains. This suggests that the mantle beneath the Cretaceous forearc was warmer than for normal forearcs. Subduction of the Farallon-Izanagi ridge may have caused the warm mantle.

Early Cretaceous Sr‐rich silicic magmatism by slab melting in the Kitakami Mountains, northeast Japan

Content and behavior of fluorine in Japanese quaternary volcanic rocks and petrogenetic application

Available data on the mode of occurrence, bulk chemical composi­ tions, and mineral textures of ultramafic rocks (ultrabasic rocks plus \"normatively\" ultramafic rocks (normative olivine+pyroxenes+ilmenite>70%)) in the Late Prot­ erozoic high-grade Lutzow-Holm Complex are summarized. The bulk chemical compositions suggest that most of them are derived from various parts of layered gabbro. Some are troctolitic, being composed mainly of normative olivine and plagioclase without pyroxenes. The mode of occurrence in the field suggests that they were tectonically fractured and emplaced in the metasedimentary gneisses. Their occurrence mainly in the western part of the Li.itzow-Holm Complex suggests that original gabbroid masses were present in the presently missing part (ocean floor?) between the adjacent Li.itzow-Holm and Yamato-Belgica Complexes. The mineral textures, especially those of garnet and spinel, suggest changing meta­ morphic conditions within the spinel-amphibole lherzolite facies during prograde recrystallization of the rocks; from earlier relatively high-pressure and low-temper­ ature conditions to later lower-pressure and higher-temperature conditions.

Satoshi Kanisawa

Papers

Two subgroups of A-type granites in the coastal area of Zhejiang and Fujian Provinces, SE China: age and geochemical constraints on their petrogenesis

Early Cretaceous Sr‐rich silicic magmatism by slab melting in the Kitakami Mountains, northeast Japan

Content and behavior of fluorine in Japanese quaternary volcanic rocks and petrogenetic application

Mode of occurrence, bulk chemical compositions, and mineral textures of ultramafic rocks in the Lutzow-Holm Complex, East Antarctica

Content and behaviour of fluorine in granitic rocks, Kitakami Mountains, northeast Japan