Fifteen andesite-dacite stratovolcanoes on the volcanic front of a single segment of the Andean arc show along-arc changes in isotopic and elemental ratios that demonstrate large crustal contributions to magma genesis. All 15 centers lie 90 km above the Benioff zone and 280±20 km from the trench axis. Rate and geometry of subduction and composition and age of subducted sediments and seafloor are nearly constant along the segment. Nonetheless, from S to N along the volcanic front (at 57.5% SiO2) K2O rises from 1.1 to 2.4 wt %, Ba from 300 to 600 ppm, and Ce from 25 to 50 ppm, whereas FeO*/MgO declines from >2.5 to 1.4. Ce/Yb and Hf/Lu triple northward, in part reflecting suppression of HREE enrichment by deep-crustal garnet. Rb, Cs, Th, and U contents all rise markedly from S to N, but Rb/Cs values double northward — opposite to prediction were the regional alkali enrichment controlled by sediment subduction. K/Rb drops steeply and scatters greatly within many (biotite-free) andesitic suites. Wide diversity in Zr/Hf, Zr/Rb, Ba/Ta, and Ba/La within and among neighboring suites (which lack zircon and alkali feldspar) largely reflects local variability of intracrustal (not slab or mantle) contributions. Pb-isotope data define a limited range that straddles the Stacey-Kramers line, is bracketed by values of local basement rocks, in part plots above the field of Nazca plate sediment, and shows no indication of a steep (mantle+sedimentary) Pb mixing trend. 87Sr/86Sr values rise northward from 0.7036 to 0.7057, and 143Nd/144Nd values drop from 0.5129 to 0.5125. A northward climb in basal elevation of volcanic-front edifices from 1350 m to 4500 m elevation coincides with a Bougueranomaly gradient from −95 to −295 mgal, interpreted to indicate thickening of the crust from 30–35 km to 50–60 km. Complementary to the thickening crust, the mantle wedge beneath the front thins northward from about 60 km to 30–40 km (as slab depth is constant). The thick northern crust contains an abundance of Paleozoic and Triassic rocks, whereas the proportion of younger arc-intrusive basement increases southward. Primitive basalts are unknown anywhere along the arc. Base-level isotopic and chemical values for each volcano are established by blending of subcrustal and deep-crustal magmas in zones of melting, assimilation, storage and homogenization (MASH) at the mantle-crust transition. Scavenging of mid-to upper-crustal silicic-alkalic melts and intracrustal AFC (prominent at the largest center) can subsequently modify ascending magmas, but the base-level geochemical signature at each center reflects the depth of its MASH zone and the age, composition, and proportional contribution of the lowermost crust.

Crustal contributions to arc magmatism in the Andes of Central Chile

Anatomy of a Cambrian suture in Gondwana: Pacific-type orogeny in southern India?

In the mid-Cretaceous, around 105±5 Ma, the tectonic regime in New Zealand changed significantly. From the Permian to the Early Cretaceous most rocks formed under the influence of convergent margin tectonics and comprise incomplete remnants of magmatic arcs, forearc basins, trench slope basins, and accretionary complexes. This tectonic pattern was ended by the approach and collision of the spreading ridge between the Phoenix and Pacific plates. Crustal extension, leading eventually to fragmentation of the continental crust, commenced immediately after ridge collision. Precollisional and syncollisional events deformed and thickened the accretionary prism, with concurrent folding and thrusting of more arcward elements. Cretaceous granulites in southwest New Zealand are thought to result from the closure of a small backarc basin or marginal sea slightly earlier than the main ridge collision. Postcollisional block faulting led to thick nonmarine sedimentation in new basins that cut across old terrane boundaries, except in the outboard area where a thick wedge of marine rocks was deposited over the subsiding accretionary prism. After oblique ridge trench collision along the New Zealand margin, the Phoenix-Pacific Ridge propagated to the southwest to link with zones of incipient spreading in the Tasman Sea and south of Australia. Subsequently, the eastern part of the ridge continued to migrate south with increasing offset along the Udinsev Fracture Zone. In the Late Cretaceous a duplicate ridge developed. The southern branch continued to move southward and collided with the trench west of the Antarctic Peninsula in the Cenozoic. The northern branch is the extant East Pacific Ridge.

Cretaceous geotectonic patterns in the New Zealand Region

This study assesses temperatures of formation of common granulites by combining experimental constraints on the P±T stability of granulite-facies mineral associations with a garnet± orthopyroxene (Grt±Opx) thermobarometry scheme based on Al-solubility in Opx, corrected for late Fe±Mg exchange. We applied this scheme to 414 granulites of mafic, intermediate and aluminous bulk compositions. Our findings suggest that granulites are much hotter than traditionally assumed and that the P±T conditions of the amphibolite±granulite transition portrayed in current petrology textbooks are significant underestimates by over 100 C. For aluminous and intermediate granulites, mean corrected temperatures based on our method are 890 17 and 841 11 C, respectively (uncertainties reported as 95% confidence limits on the mean), consistent with minimum temperatures for orthopyroxene production by fluid-absent partial melting in these bulk compositions. In contrast, mean temperatures based on Grt±Opx Fe±Mg exchange equilibria, using the same thermodynamic data, are 732 22 and 723 11 C, respectively, well below the minimum temperatures for Opx stability. For mafic granulites, the mean corrected temperature using our method is 816 12 C, similar to the mean temperature of 793 13 C from Fe±Mg exchange. Reasons for the differences between the mafic granulites and aluminous±intermediate granulites are unclear but may be due to the lower Al concentrations in Opx in the mafic rocks and possible deficiencies in the thermodynamic modelling of these low concentrations. We discuss a number of well-known granulite terrains in the context of our findings, including the Adirondacks, the Acadian granulites of New England, the incipient charnockites of southern India and Sri Lanka, and the Kerala Khondalite Belt. Our findings carry implications for thermotectonic models of granulite formation. A computer program to perform our thermobarometry calculations, RCLC, is available from the Journal of Petrology website at http://www.petrology.oupjournals.org or from the authors at http://www.geo.ucalgary.ca/ pattison/drm_pattison-rclc.htm.

Temperatures of Granulite-facies Metamorphism: Constraints from Experimental Phase Equilibria and Thermobarometry Corrected for Retrograde Exchange

The Early Cretaceous Separation Point batholith of the South Island, New Zealand, represents the final magmatic stage of an extensive arc system located on the SW Pacific margin of Gondwana during the Mesozoic. The batholith consists of Na-rich, alkali-calcic diorite to biotite-hornblende monzogranite. The rocks are distinct from calc-alkaline subduction-related granitoids, but comparable with those of adakite and Archaean trondhjemite-tonalite-dacite suites. Primitive Sr and Nd isotopic ratios and the absence of inherited zircon, indicate that the granitoids experienced little, if any, interaction with felsic crust. Their geochemistry is consistent with melting of a basaltic protolith of amphibolite mineralogy, either young, hot, subducted oceanic crust or newly underplated material beneath a thickened continental arc. The latter model is preferred because Separation Point rocks do not posess MORB isotopic characteristics, and cannot be explained as mixtures of MORB-melt and continental crust. Most likely it involves melting of basal arc material in response to the collision and thrusting of the arc beneath the continental margin following subduction of a back-arc basin. On the basis of strong geochemical similarities, the Early Cretaceous Western Fiordland Orthogneiss of SW New Zealand is considered to be the lower crustal equivalent of the Separation Point batholith.

Geochemistry of the Cretaceous Separation Point Batholith, New Zealand : granitoid magmas formed by melting of mafic lithosphere

U-Pb isotopic analyses of zircons from a distinctive suite of previously undated granulite facies metaplutonic rocks, here termed the Western Fiordland Orthogneiss (WFO), in Fiordland, southwest New Zealand, indicate synkinematic magmatic emplacement between ∼120 and 130 Ma ago. These rocks were previously interpreted as possibly being of Precambrian age. Initial Pb and Sr ratios are consistent with arc/subduction related magmagenesis with little or no involvement of ancient continental crust. Subsequent high pressure (>12 kb) metamorphism of the WFO may reflect a major collision event involving crustal thickening by overthrusting of a >15 km thick sequence. Metamorphism ceased ≤116 Ma ago based on206Pb/238U ages of zircon from a retrogressed granulite. U-Pb isotopic analysis of apatite, along with previously published Rb/Sr mineral ages, indicate that final uplift and cooling to <300–400° C was largely completed by ∼90 Ma. The average uplift rate during this period is inferred to have been in excess of 1 mm/yr. Unmetamorphosed gabbronorites of the Darran Complex in eastern Fiordland, inferred by some investigators to be the granulite protolith, yield concordant U/Pb zircon ages of 137±1 Ma. U-Pb ages of apatite, and previously published K/Ar mineral ages indicate that these rocks experienced a rapid and simple cooling history lasting only a few million years. The high-grade WFO and unmetamorphosed Darran Complex are now separated by a profound structural break. However, the ages and similarities in initial Pb and Sr isotopic ratios suggest that both suites are products of the same Early Cretaceous cycle of subduction-related magmatism. The timing of Early Cretaceous magmatism and metamorphism, collision and resultant crustal thickening, and subsequent great uplift and erosion in Fiordland has important implications for terrane accretion and the evolution of relative plate motions along the New Zealand segment of the Gondwana margin.

Western Fiordland orthogneiss: Early Cretaceous arc magmatism and granulite facies metamorphism, New Zealand

Regionally extensive two-pyroxene granulites in Fiordland, southwest New Zealand, are products of metamorphism of a suite of anhydrous magmas which crystallized two pyroxenes. The granulite protolith (igneous charnockitic rock) synkinematically intruded metasediment and other orthogneiss in an Early Cretaceous subduction-related magmatic arc, and during cooling experienced deformation-induced recrystallization to form granoblastic gneiss. The granulites occur side by side with coeval rocks of amphibolite facies. Mineral zoning and textural relationships in both granulites and amphibolite facies rocks provide evidence of two distinct periods of crystallization: 1) an early high temperature, comparatively low pressure event accompanying magmatic intrusion (andalusite-sillimanite facies series recorded locally in the country rock), followed by 2) high pressure metamorphism under conditions of ∼650°–700° C at ∼12–13 kbar. Garnet granulite locally overprinted earlier formed two-pyroxene granulite during the latter event. The pressure increase (∼6 kbar) between the two events is attributed to crustal thickening by overthrusting, and is equivalent to unloading of a ∼20 km thick slab over rocks already buried at mid-crustal depths. Both events occurred over a < 20 m.y. interval, between the time of magmatic emplacement of the granulite protolith and uplift-controlled final cooling of the terrain. The Phanerozoic granulites in Fiordland share some petrologic similarities with Precambrian granulite terrains, suggesting that at least some aspects of the former may serve as a useful model for development of the latter.

Origin and metamorphic history of an Early Cretaceous polybaric granulite terrain, Fiordland, southwest New Zealand

Sm-Nd and Rb-Sr isotopic analyses are reported for granulite facies orthogneisses from Fiordland southwest New Zealand. Whole-rock samples define a Rb-Sr isochron age of 120±15 Ma and an initial 87Sr/86Sr ratio of 0.70391±4. ɛ
Nd values (at 120 Ma) show a relatively wide range of from −0.4 to 2.7 indicating decoupling of Sr-Nd isotope systems. Associated ultramafic rocks have initial 87Sr/86Sr ratios of from 0.70380 to 0.70430 and ɛ
Nd values of from 0.1 to 3.0. The different initial ratios suggest that the various intrusions, although contemporaneous, were not derived through fractionation of a single parent magma. A metasedimentary enclave incorporated during emplacement of the granulitic rocks preserves a Proterozoic isotopic signature with a measured ɛ
Nd(0) value of −10.2, 87Sr/86Sr ratio of 0.73679 and a T
Nd provenance age of 1490 Ma. The Rb-Sr whole rock age of the granulites is the same as obtained from recent U-Pb zircon dating (Mattinson et al. 1986) and is interpreted as the time of magmatic emplacement and essentially contemporaneous granulite facies metamorphism. Rb-Sr and Sm-Nd analyses of mineral systems indicate that the terrain had cooled below ∼300° C by ∼100 Ma providing further evidence that high grade metamorphism was of exceptionally short duration. Unmetamorphosed leucogabbros from the Early Cretaceous Darran Complex of eastern Fiordland have significantly higher ɛ
Nd values (3.9 to 4.6) and slightly lower 87Sr/ 86Sr (0.70373 to 0.70386) than the western Fiordland granulites. This indicates that the western and eastern Fiordland complexes are not correlative although both have geochemical similarities to Phanerozoic calc-alkaline island-arc suites. The Fiordland granulites are LREE enriched (LaN/ YbN=12 to 40) and have trace element characteristics (e.g. high K/Rb and low Rb/Sr ratios) typical of many Rb-depleted Precambrian granulite terrains. The Fiordland trace element trends, however are attributed to magmatic, not metamorphic processes, reflecting the character of the Early Cretaceous magma sources. The range of ɛ
Nd values, but uniform initial 87Sr/86Sr of the western Fiordland granulites is consistent with derivation of the parent Early Cretaceous magmas at least in part from a LREE enriched, low Rb/Sr protoliths of mid-to late-Paleozoic age. Partial melting of this protolith occurred during or immediately preceding a period of great crustal thickening culminating in rapid thickening of existing crust by ∼20 km following emplacement of the granulitic rocks. The rapid crustal thickening was probably a consequence of a collisional event in which an Early Cretaceous magmatic arc was over-ridden by one or more thrust sheets.

J. Y. Bradshaw

Papers

Western Fiordland orthogneiss: Early Cretaceous arc magmatism and granulite facies metamorphism, New Zealand

Origin and metamorphic history of an Early Cretaceous polybaric granulite terrain, Fiordland, southwest New Zealand

Sm-Nd and Rb-Sr isotopic and geochemical systematics in Phanerozoic granulites from Fiordland, southwest New Zealand