Emplacement and deformation of the Wyangala Batholith,New South Wales

The Wyangala Batholith, in the Lachlan Fold Belt of New South Wales, is pre‐tectonic with respect to the deformation that caused the foliation in the granite, and was emplaced during a major thermal event, perhaps associated with dextral shearing, during the Late Silurian to Early Devonian Bowning Orogeny. This followed the first episode of folding in the enclosing Ordovician country rocks. Intrusion was facilitated by upward displacement of fault blocks, with local stoping. Weak magmatic flow fabrics are present. After crystallization of the granite, a swarm of mafic dykes intruded both the granite and country rock, possibly being derived from the same tectonic regime responsible for emplacement of the Wyangala Batholith. A contact aureole surrounding the granite contains cordierite‐biotite and cordierite‐andalusite assemblages. Slaty cleavage produced in the first deformation was largely obliterated by recrystallization in the contact aureole.

Postdating granite emplacement and basic dyke intrusion, a second regional deformation was accompanied by regional metamorphism ranging from lower greenschist to albite‐epidote‐amphibolite facies, and produced tectonic foliations, termed S and C, in the granite, and a foliation, S₂, in the country rocks. Contact metamorphic rocks underwent retrogressive regional metamorphism at this time. S formed under east‐west shortening and vertical extension, concurrently with S₂. C surfaces probably formed concurrently with S and indicate reverse fault motion on west‐dipping ductile shear surfaces. The second deformation may be related to Devonian or Early Carboniferous movement on the Copperhannia Thrust east of the Wyangala Batholith.     

“Normal” and “inverted” metamorphic isograds and their relation to syntectonic proterozoic batholiths in the Wopmay Orogen, Northwest Territories, Canada

Three progressive metamorphic suites are developed in pelitic rocks of the northern Wopmay Orogen. Two suites are related to the Hepburn Batholith and one to the Wentzel Batholith. All three suites are cut by post-metamorphic wrench faults, some of which have significant vertical displacement. The structural relief so provided reveals that medium-and high-grade isograds associated with the Hepburn Batholith dip inward towards the batholith and are thus “hot-side-up”. Isograds associated with the Wentzel Batholith dip away from the batholith and are thus “hot-side-down”. It is concluded that Hepburn Batholith has the form of the flattened funnel fed from depth, and that Wentzel Batholith is the arched roof of an intrusive complex of unknown shape at depth.     

Geochronology and geochemistry of high‐pressure granulites of the Arthur River Complex,Fiordland, New Zealand: Cretaceous magmatism and metamorphism on the palaeo‐Pacific Margin

The Arthur River Complex is a suite of gabbroic to dioritic orthogneisses in northern Fiordland, New Zealand. The Arthur River Complex separates rocks of the Median Tectonic Zone, a Mesozoic island arc complex, from Palaeozoic rocks of the palaeo‐Pacific Gondwana margin, and is itself intruded by the Western Fiordland Orthogneiss. New SHRIMP U/Pb single zircon data are presented for magmatic, metamorphic and deformation events in the Arthur River Complex and adjacent rocks from northern Fiordland. The Arthur River Complex orthogneisses and dykes are dominated by magmatic zircon dated at 136–129 Ma. A dioritic orthogneiss that occurs along the eastern margin of the Complex is dated at 154.4 ± 3.6 Ma and predates adjacent plutons of the Median Tectonic Zone. Rims on zircon cores from this sample record a thermal event at c. 120 Ma, attributed to the emplacement of the Western Fiordland Orthogneiss. Migmatitic Palaeozoic orthogneiss from the Arthur River Complex (346 ± 6 Ma) is interpreted as deformed wall rock. Very fine rims (5–20 µm) also indicate a metamorphic age of c. 120–110 Ma. A post‐tectonic pegmatite (81.8 ± 1.8 Ma) may be related to phases of crustal extension associated with the opening of the Tasman Sea. The Arthur River Complex is interpreted as a batholith, emplaced at mid‐crustal levels and then buried to deep crustal levels due to convergence of the Median Tectonic Zone arc and the continental margin.     

P–T–t–d evolution of orogenic middle crust of the Roc de Frausa Massif (Eastern Pyrenees): a result of horizontal crustal flow and Carboniferous doming?

Structural, petrological and textural studies are combined with phase equilibria modelling of metapelites from different structural levels of the Roc de Frausa Massif in the Eastern Pyrenees. The pre‐Variscan lithological succession is divided into the Upper, Intermediate and Lower series by two orthogneiss sheets and intruded by Variscan igneous rocks. Structural analysis reveals two phases of Variscan deformation. D1 is marked by tight to isoclinal small‐scale folds and an associated flat‐lying foliation (S1) that affects the whole crustal section. D2 structures are characterized by tight upright folds facing to the NW with steep NE–SW axial planes. D2 heterogeneously reworks the D1 fabrics, leading to an almost complete transposition into a sub‐vertical foliation (S2) in the high‐grade metamorphic domain. All structures are affected by late open to tight, steeply inclined south‐verging NW–SE folds (F3) compatible with steep greenschist facies dextral shear zones of probable Alpine age. In the micaschists of the Upper series, andalusite and sillimanite grew during the formation of the S1 foliation indicating heating from 580 to 640 °C associated with an increase in pressure. Subsequent static growth of cordierite points to post‐D1 decompression. In the Intermediate series, a sillimanite–biotite–muscovite‐bearing assemblage that is parallel to the S1 fabric is statically overgrown by cordierite and K‐feldspar. This sequence points to ~1 kbar of post‐D1 decompression at 630–650 °C. The Intermediate series is intruded by a gabbro–diorite stock that has an aureole marked by widespread migmatization. In the aureole, the migmatitic S1 foliation is defined by the assemblage biotite–sillimanite–K‐feldspar–garnet. The microstructural relationships and garnet zoning are compatible with the D1 pressure peak at ~7.5 kbar and ~750 °C. Late‐ to post‐S2 cordierite growth implies that F2 folds and the associated S2 axial planar leucosomes developed during nearly isothermal decompression to <5 kbar. The Lower series migmatites form a composite S1–S2 fabric; the garnet‐bearing assemblage suggests peak P–T conditions of >5 kbar at suprasolidus conditions. Almost complete consumption of garnet and late cordierite growth points to post‐D2 equilibration at <4 kbar and <750 °C. The early metamorphic history associated with the S1 fabric is interpreted as a result of horizontal middle crustal flow associated with progressive heating and possible burial. The upright F2 folding and S2 foliation are associated with a pressure decrease coeval with the intrusion of mafic magma at mid‐crustal levels. The D2 tectono‐metamorphic evolution may be explained by a crustal‐scale doming associated with emplacement of mafic magmas into the core of the dome.     

Regional and contact metamorphism within the Moy Intrusive Complex,Grampian Highlands,Scotland

In central Scotland, the Moy Intrusive Complex consists of (1) the Main Phase — syntectonic peraluminous granodiorite to granite emplaced at c. 455 Ma, intruded by (2) the Finglack Alaskite — post-tectonic leucocratic granite emplaced at 407+/-5 Ma. The Main Phase was emplaced into country rocks at amphibolite facies temperatures. Rb-Sr dates and a compositional spectrum of decreasing celadonite content in Main Phase muscovite suggest the persistence of c. 550° C temperatures for c. 30 Ma but with a declining pressure regime, i.e. isothermal uplift. The Finglack Alaskite was intruded at high structural level, leading to the development of a contact metamorphic aureole in the Main Phase. The thermal effects of contact metamorphism include intergrowths of andalusite, biotite and feldspar in pseudomorphs after muscovite. This is associated with recrystallized granoblastic quartz. Muscovite breakdown and reaction with adjacent biotite, quartz and feldspar, i.e. a function of local mineral assemblage rather than bulk rock composition, is postulated to explain the occurrence of metamorphic andalusite in a granitoid rock.The Main Phase pluton of the Moy Intrusive Complex lies within a NNE trending belt of c. 450 Ma Caledonian tectonic and magmatic activity paralleling the Moine Thrust, and extending from northern Scotland to the Highland Boundary Fault. Syntectonic S-type magmatism with upper crustal source areas implies crustal thickening and suggests an intracratonic orogeny.     

Petrochemistry of the claret creek ring complex,northeast Queensland

The Claret Creek Ring Complex is one of several calc‐alkaline ring complexes in a Carboniferous epizonal batholith emplaced into continental crust at the junction of the Precambrian Georgetown Inlier and the adjacent Palaeozoic Tasman Geo‐syncline, northeast Queensland. Rhyolite ash‐flow sheets plus rhyolite and dacite ring dykes are intruded by two comagmatic central stocks of microgranite and grano‐diorite‐tonalite. The complex may be chemically distinguished from the surrounding, contemporaneous batholith by its low K/Na, Rb/Sr and Th/K ratios. The origin and variation of its magmas is explained by invoking progressive partial melting of low K/Na basaltic andesites. Close relatives to the magma source‐rock are preserved as microdiorite xenoliths, which have contaminated their host granodiorite‐tonalite stock.     

Granulite facies thermal aureoles and metastable amphibolite facies assemblages adjacent to the Western Fiordland Orthogneiss in southwest Fiordland, New Zealand

In southwest New Zealand, a suite of felsic diorite intrusions known as the Western Fiordland Orthogneiss (WFO) were emplaced into the mid to deep crust and partially recrystallized to high‐P (12 kbar) granulite facies assemblages. This study focuses on the southern most pluton within the WFO suite (Malaspina Pluton) between Doubtful and Dusky sounds. New mapping shows intrusive contacts between the Malaspina Pluton and adjacent Palaeozoic metasedimentary country rocks with a thermal aureole ～200–1000 m wide adjacent to the Malaspina Pluton in the surrounding rocks. Thermobarometry on assemblages in the aureole indicates that the Malaspina Pluton intruded the adjacent amphibolite facies rocks while they were at depths of 10–14 kbar. Similar P–T conditions are recorded in high‐P granulite facies assemblages developed locally throughout the Malaspina Pluton. Palaeozoic rocks more than ～200–1000 m from the Malaspina Pluton retain medium‐P mid‐amphibolite facies assemblages, despite having been subjected to pressures of 10–14 kbar for > 5 Myr. These observations contradict previous interpretations of the WFO Malaspina Pluton as the lower plate of a metamorphic core complex, everywhere separated from the metasedimentary rocks by a regional‐scale extensional shear zone (Doubtful Sound Shear Zone). Slow reaction kinetics, lack of available H₂O, lack of widespread penetrative deformation, and cooling of the Malaspina Pluton thermal anomaly within c. 3–4 Myr likely prevented recrystallization of mid amphibolite facies assemblages outside the thermal aureole. If not for the evidence within the thermal aureole, there would be little to suggest that gneissic rocks which underlie several 100 km² of southwest New Zealand had experienced metamorphic pressures of 10–14 kbar. Similar high‐P metamorphic events may therefore be more common than presently recognized.     

Metasomatic albitites and related biotite-rich schists from a low-pressure polymetamorphic terrane, Snake Creek Anticline, Mount Isa Inlier, north-eastern Australia: microstructures and P–T–d paths

Rocks of the Snake Creek Anticline are mainly pelitic schists, psammitic schists and quartzites that were metamorphosed during multiple high‐T/low‐P events extending from D1 to D5, with the metamorphic peak occurring late to post‐D3. Albitites are widespread, but are concentrated in five areas. They are typically fine‐ to medium‐grained, and consist of albite, with or without combinations of quartz, biotite, staurolite, cordierite, garnet, andalusite, sillimanite, kyanite, gedrite and tourmaline. From the presence or absence of albite inclusions in porphyroblasts, the albitites are interpreted as forming early in the D3 event as a result of infiltration of external fluids. Psammitic schists and quartzites were preferentially altered, but pelitic schists were also albitized in localities where the alteration was more extreme, with the replacement of muscovite total and the replacement of quartz and biotite variable. Structural controls on albitization include fracturing and syn‐D3 shear zones in fold hinges. Biotite schists with abundant porphyroblasts (combinations of staurolite, garnet, andalusite and cordierite) occur adjacent to albitites, and it is argued that they formed by the addition of Fe and Mg sourced from the albitites. In several albitite‐rich areas, cordierite grew early in D3 and was partly or entirely replaced during or after D3 by combinations of biotite, andalusite, tourmaline, staurolite and sillimanite. A postulated P–T–d path involved an increase in pressure (with or without a decrease in temperature) subsequent to early D3 albitization, followed by an increase in temperature up to the metamorphic peak (late D3 to early D4. The metamorphism was contemporary in part with the emplacement of the Williams Batholith (c. 1550–1500 Ma), which probably supplied the Na‐rich fluids.     

Metamorphism at a late Mesozoic accretionary margin: a study from the Coast Belt of the North American Cordillera

The late Mesozoic and Cenozoic metamorphic evolution of the western North American continental margin is recorded in a belt of homogeneous metapelitic rocks, the Kluane metamorphic assemblage (KMA), in the northern Coast Belt of Yukon Territory. A record of Late Cretaceous medium‐pressure and ‐temperature (c. 7 kbar, 500 °C) metamorphism, M1, is preserved in Ca‐rich garnet and Na‐rich plagioclase cores in rocks that were little affected by later events. M1 was synchronous with mylonitization and is attributed to accretion of the KMA to the ancient continental margin. Isothermal decompression during rapid uplift was followed by early Eocene emplacement of the Ruby Range Batholith (RRB), part of a magmatic arc produced by subduction of the Kula plate. The intrusion of the RRB led to a contact metamorphic overprint, M2, producing a 5–6 km wide aureole in which the grade ranges from subgarnet zone to garnet–cordierite–K‐feldspar zone. Pressure and temperature estimates for M2, calculated from mineral equilibria, are 3.5–4.5 kbar and 530–720 °C, generally consistent with the stability limits of the observed mineral assemblages. Comparison of mineral assemblages and P–T conditions in the KMA with those in the Mclaren Glacier metamorphic belt in Alaska does not support the correlation of the two metamorphic sequences. This weakens the hypothesis proposing 400 km of dextral slip along the Denali fault zone.     

Geologic and Tectonic Setting of the Yozgat Batholith,Northern Central Anatolian Crystalline Complex,Turkey

The Yozgat Batholith lies along the northern edge of the Central Anatolian Crystalline Complex in Central Anatolia, Turkey. The batholith intruded the Paleozoic-Mesozoic metamorphics and Cretaceous ophiolitic mélange, and was nonconformably overlain by latest Maastrichtian-Paleocene and/or Eocene clastics, carbonates, and volcanics. The batholith itself may be subdivided into several mappable subunits bounded by Cretaceous ophiolitic mélange, Eocene cover, and/or faults.

Major- and trace-element as well as REE analyses of the subunits indicate that the granitoids of the Yozgat Batholith are principally metaluminous monzogranites, of subalkaline-calc-alkaline character, except for the peraluminous leucogranitoids of the Yozgat subunit. The granitoids were derived by thickening of the continental crust and related partial melting; the thickening was caused by emplacement of ophiolitic nappes during collisional events.     

Metamorphic and tectonic evolution of a structurally continuous blueschist‐to‐Barrovian terrane,Sivrihisar Massif,Turkey

Metamorphic terranes comprised of blueschist facies and regional metamorphic (Barrovian) rocks in apparent structural continuity may represent subduction complexes that were partially overprinted during syn‐ to post‐subduction heating or may be comprised of unrelated tectonic slices. An excellent example of a composite blueschist‐to‐Barrovian terrane is the southern Sivrihisar Massif, Turkey. Late Cretaceous blueschist facies rocks are dominated by marble characterized by rod‐shaped calcite pseudomorphs after aragonite and interlayered with blueschist that contains eclogite and quartzite pods. Barrovian rocks, which have ⁴⁰Ar/³⁹Ar white mica ages that are >20 Myr younger than those of the blueschists, are also dominated by marble, but rod‐shaped calcite has been progressively recrystallized into massive marble within a ～200‐m transition zone. Barrovian marble is interlayered with quartzite and schist in which isograds are closely spaced and metamorphic conditions range from chlorite to sillimanite zone over ～1 km present‐day structural thickness. Andalusite, kyanite and prismatic sillimanite are present in muscovite‐rich quartzite; in one location, all three are in the same rock. Andalusite pre‐dates Barrovian metamorphism, kyanite is both pre‐ and syn‐Barrovian and sillimanite is entirely Barrovian. Muscovite with phengitic cores and relict kyanite in quartzite below the staurolite‐in isograd are evidence for pre‐Barrovian subduction metamorphism preserved at the low‐T end of the Barrovian domain; above the staurolite isograd, all evidence for subduction metamorphism has been erased. Some regional metamorphism may have occurred during exhumation, as indicated by syn‐kinematic high‐T minerals defining the fabric of L‐tectonite. Quartz microstructures in lineated quartzite reveal a strong constrictional fabric that may have formed in a transtensional bend in the plate boundary. Transtension accounts for the closely spaced isograds and development of a strong constrictional fabric during exhumation.     

Shape and intrusion style of the Coastal Batholith, Peru

The Coastal Batholith of Peru extends over 1600 km parallel to the coast along the Andean trend. Gravity profiles on three traverses across the batholith indicate the geometry is essentially that of a flat slab with average thickness from 2.0–3.2 km, and a thick root 4–10 km wide to the west. Granitic material does not extend to depths greater than 3 km below sea level datum.This study supports recent gravity work which indicates plutons are commonly thin, 5 km or less in thickness. Detailed mapping in the Lima segment of the Coastal Batholith reveals thin plutons where space was made dominantly by downward displacement via floor depression. However, early roof uplift also created some space. Stoping occurs but is not a major space maker. Floor depression may be modelled by cantilever or piston mechanisms and although the strong marginal deformation with mylonites, tuffisites, microbreccia, faults and shear zones suggests the piston model best describes the mechanism of emplacement of much of the Coastal Batholith some space was probably made by a cantilever mechanism. In brief, space making processes involved early roof uplift and regional doming, then floor depression mainly by piston and probably subsidiary cantilever mechanisms and, finally, local stoping producing the cut-out rectilinear nature of the batholith.The Coastal Batholith formed on shallow partial melting of hydrous basaltic marginal basin rocks between 5 and 10 km. Floor depression occurred as the crustal column foundered into an actively deflating layer of partial melt. This is an efficient space making process and is limited here to shallow levels of the upper crust only. The melts ascended to within 2 or 3 km of the surface, up dyke-like conduits then spread horizontally to form tabular plutons.     

Radiometric evidence for the age of emplacement and cooling of the Murrumbidgee Batholith

Most of the rocks of the Murrumbidgee Batholith have a Rb‐Sr age of 424 ± 2 m.y. This is considered to be the time of emplacement. A small difference in the ages (4 ± 2 m.y.) between the northern and southern parts of the batholith is attributed to thermal effects caused by a slightly later time of emplacement of some of the intrusions or to a short cooling interval. Final intrusive activity ended by 414 ± 4 m.y. Younger mineral ages for some intrusions are related to later local meta‐morphic effects.     

Syn-deformational migmatites and magmatic-arc metamorphism in the Xolapa Complex, southern Mexico

The Xolapa Complex (XC) is the largest plutonic and metamorphic mid‐crustal basement unit in Mexico and represents an ancient continental magmatic‐arc. A complete range from metatexite to diatexite migmatitic structures has been produced during a single high‐grade metamorphic event. However, structural relics reveal the existence of early Cpx + Pl + Qtz ± Opx and Grt + Opx + Pl + Qtz ± Cpx pre‐migmatitic metamorphic assemblages. Field relationships and microstructural observations allow us to constrain five pre‐, syn‐ and post‐migmatitic deformational phases. It is argued that migmatitic structures and minor anatectic granites were developed during ductile recumbent folding and shear structures related to the D2–D3 phases. Late post‐migmatitic ductile‐brittle deformation is evidenced by the development of NNE trending transpressional thrusting (D4), and E–W left‐lateral mylonitic shear zones (D5). Biotite‐breakdown melting in felsic rocks and amphibole‐breakdown melting in mafic rocks, as well as geothermobarometric results, indicate that metamorphism took place at temperatures from 830 to 900 °C and pressures ranging from ≥6.3 to 9.5 kbar. Late migmatitic assemblages equilibrated in the highest temperature range along a clockwise P–T path. The relationships between the large diversity of migmatitic structures and the progressive production of melt suggest that feedback relations prevailed as a time‐marker during a contractional regime. Deformation, metamorphism, and plutonism of the XC show that this terrane evolved as a north‐east‐verging thrust system with synkinematic metamorphism and partial melting, during the Late Cretaceous – Palaeogene. The tectonothermal history of XC is analogous to a Cordilleran metamorphic magmatic‐arc formed in an accretionary tectonic framework. This new model provides constraints on the exhumation mechanism and thermal evolution of southern Mexico.     

Contact metamorphism of impure dolomitic limestone in the Boulder Aureole,Montana

Progressive metamorphism of impure dolomitic limestone in the 1.5 to 2.5 km wide contact aureole surrounding the northernmost portion of the boulder batholith has resulted in a consistent sequence of uniformly distributed zones of low-variance mineral parageneses separated by abrupt and distinctive isograds. In silica-undersaturated, aluminous marbles, the following mineral assemblages occur, in order of increasing grade: calcite-dolomite-calcic amphibole-chlorite, calcite-dolomite-calcic amphibole-chlorite-spinel, calcite-dolomite-calcic amphibole-chlorite-olivine-spinel, calcite-dolomite-chlorite-olivine-spinel, calcite-dolomite-olivine-spinel. The spatial distribution of parageneses and the occurrence of low-variance parageneses indicate buffering of the pore fluid composition by the local mineral assemblages. The observed sequence of mineral reactions and the spacing of isograds is in good agreement with experimental and calculated equilibria in terms of P-T-X _{CO 2}and temperatures of equilibration inferred from calcite-dolomite geothermometry, which range from 435 to 607 °C across the aureole.Microprobe analyses of coexisting minerals indicate attainment of exchange equilibrium. Calcic amphibole and chlorite coexisting with calcite and dolomite become progressively more aluminous with increasing grade; calcic amphibole changes rapidly from Al-poor tremolite to pargasite, while Al^IV in Cte increases from 2.0 to 2.3 atoms per 8 tetrahedral sites. Observed low-variance assemblages fix the activities of calcic amphibole and chlorite end-member components as a function of P and T, and hence the systematic compositional variation in these phases is not an independent variable, but is controlled by the local mineral assemblage.     

The Berridale Wrench fault: A major structure in the Snowy Mountains of New South Wales

One of the most significant, but poorly understood, tectonic events in the east Lachlan Fold Belt is that which caused the shift from mafic, mantle‐derived calc‐alkaline/shoshonitic volcanism in the Late Ordovician to silicic (S‐type) plutonism and volcanism in the late Early Silurian. We suggest that this chemical/isotopic shift required major changes in crustal architecture, but not tectonic setting, and simply involved ongoing subduction‐related magmatism following burial of the pre‐existing, active intraoceanic arc by overthrusting Ordovician sediments during Late Ordovician — Early Silurian (pre‐Benambran) deformation, associated with regional northeast‐southwest shortening. A review of ‘type’ Benambran deformation from the type area (central Lachlan Fold Belt) shows that it is constrained to a north‐northwest‐trending belt at ca 430 Ma (late Early Silurian), associated with high‐grade metamorphism and S‐type granite generation. Similar features were associated with ca 430 Ma deformation in east Lachlan Fold Belt, highlighted by the Cooma Complex, and formed within a separate north‐trending belt that included the S‐type Kosciuszko, Murrumbidgee, Young and Wyangala Batholiths. As Ordovician turbidites were partially melted at ca 430 Ma, they must have been buried already to ～20 km before the ‘type’ Benambran deformation. We suggest that this burial occurred during earlier northeast‐southwest shortening associated with regional oblique folds and thrusts, loosely referred to previously as latitudinal or east‐west structures. This event also caused the earliest Silurian uplift in the central Lachlan Fold Belt (Benambran highlands), which pre‐dated the ‘type’ Benambran deformation and is constrained as latest Ordovician — earliest Silurian (ca 450–440 Ma) in age. The south‐ to southwest‐verging, earliest Silurian folds and thrusts in the Tabberabbera Zone are considered to be associated with these early oblique structures, although similar deformation in that zone probably continued into the Devonian. We term these ‘pre’‐ and ‘type’‐Benambran events as ‘early’ and ‘late’ for historical reasons, although we do not consider that they are necessarily related. Heat‐flow modelling suggests that burial of ‘average’ Ordovician turbidites during early Benambran deformation at 450–440 Ma, to form a 30 km‐thick crustal pile, cannot provide sufficient heat to induce mid‐crustal melting at ca 430 Ma by internal heat generation alone. An external, mantle heat source is required, best illustrated by the mafic ca 430 Ma, Micalong Swamp Igneous Complex in the S‐type Young Batholith. Modern heat‐flow constraints also indicate that the lower crust cannot be felsic and, along with petrological evidence, appears to preclude older continental ‘basement terranes’ as sources for the S‐type granites. Restriction of the S‐type batholiths into two discrete, oblique, linear belts in the central and east Lachlan Fold Belt supports a model of separate magmatic arc/subduction zone complexes, consistent with the existence of adjacent, structurally imbricated turbidite zones with opposite tectonic vergence, inferred by other workers to be independent accretionary prisms. Arc magmas associated with this ‘double convergent’ subduction system in the east Lachlan Fold Belt were heavily contaminated by Ordovician sediment, recently buried during the early Benambran deformation, causing the shift from mafic to silicic (S‐type) magmatism. In contrast, the central Lachlan Fold Belt magmatic arc, represented by the Wagga‐Omeo Zone, only began in the Early Silurian in response to subduction associated with the early Benambran northeast‐southwest shortening. The model requires that the S‐type and subsequent I‐type (Late Silurian — Devonian) granites of the Lachlan Fold Belt were associated with ongoing, subduction‐related tectonic activity.     

Delamerian Glenelg tectonic zone,western Victoria: Geology and metamorphism of stratiform rocks

The Cambro‐Ordovician Glenelg tectonic zone of western Victoria is a distinctive metamorphic‐igneous segment of the Delamerian Orogenic Belt comprising two northwest‐striking regional metamorphic segments of andalusite‐sillimanite type prograding towards an axial granitic batholith. The second of five deformations (D₂) was most significant, producing isoclinal folds, transposition and a pervasive regional foliation (S₂). Southwest of the central batholith, biotite to migmatite zones contain mainly quartzo‐feldspathic rock (turbiditic metagreywacke, quartzo‐feldspathic schist and migmatite), plus less common metaquartzite and calc‐silicate rocks and minor metapelite. Metagabbro, metadolerite and amphibolite typically have the chemistry of mid‐ocean ridge basalts. Serpentinite pods and sheets were tectonically introduced to low‐grade areas. Northeast of the central batholith, quartzo‐feldspathic rock occupies the sillimanite and migmatite zones exclusively, with a regional concentration of pegmatites adjacent to the zone boundary. Gross interleaving of quartzo‐feldspathic schist, migmatite, pegmatite and muscovite‐bearing granitic rock is characteristic. Peak metamorphic conditions of 550 MPa at 640°C leading to migmatite formation were established by D₂ time and accompanied by tonalite‐granodiorite and pegmatite emplacement. Subsequently, the thermal high contracted to the northeast culminating in the more extensive syn‐, post‐D4 to pre‐D5 granitic magmatism.     

Successive hydration and dehydration of high-P mafic granofels involving clinopyroxene–kyanite symplectites, Mt Daniel, Fiordland, New Zealand

Three texturally distinct symplectites occur in mafic granofels of the Arthur River Complex at MtDaniel, Fiordland, New Zealand. These include symplectic intergrowths of clinopyroxene and kyanite, described here for the first time. Pods of mafic granofels occur within the contact aureole of the Early Cretaceous Western Fiordland Orthogneiss batholith. The pods have cores formed entirely of garnet and clinopyroxene, and rims of pseudomorphous coarse‐grained symplectic intergrowths of hornblende and clinozoisite that reflect hydration at moderate to high‐P. These hornfelsic rocks are enveloped by a hornblende–clinozoisite gneissic foliation (S1). Narrow garnet reaction zones, in which hornblende and clinozoisite are replaced by garnet–clinopyroxene assemblages, developed adjacent to fractures and veins that cut S1. Fine‐grained symplectic intergrowths of (1) clinopyroxene and kyanite and (2) clinozoisite, quartz, kyanite and plagioclase form part of the garnet reaction zones and partially replace coarse‐grained S1 hornblende and clinozoisite. The development of the garnet reaction zones and symplectites was promoted by dehydration most probably following cooling of the contact aureole. Maps of oxide weight percent and cation proportions, calculated by performing matrix corrections on maps of X‐ray intensities, are used to study the microstructure of the symplectites.     

Petrology of metapelites in the Bugaboo aureole,British Columbia,Canada

Contact metamorphism of greenschist facies Neoproterozoic turbidites by the Cretaceous Bugaboo Batholith in southeastern British Columbia has resulted in a well‐developed contact aureole. The aureole is about 1 km wide and can be divided into three main zones: (i) spotted phyllite zone, extending from the first appearance of spots of cordierite or andalusite to the last occurrence of primary chlorite; (ii) cordierite + andalusite + biotite zone, comprising hornfelses or schists with abundant porphyroblasts of cordierite and andalusite and, at higher grades, fibrolitic sillimanite; and (iii) K‐feldspar zone, characterized by hornfelses and schists that, in the inner part of this zone, are variably migmatitic. Four parts of the aureole were examined, three of which are characterized by schists, and one of which (Cobalt Lake area) is characterized by hornfelses and has exceptional exposure and comparatively unaltered rocks. Petrographic, modal, mineral‐compositional and whole rock‐compositional data were collected from the Cobalt Lake transect, allowing the prograde reaction sequence to be inferred. Notable features of the aureole at Cobalt Lake include: initial development of andalusite and plagioclase at the expense of paragonite‐rich white mica; a narrow interval across which cordierite, andalusite and biotite increase markedly at the expense of chlorite; gradual development of andalusite and biotite at the expense of cordierite and muscovite upgrade of chlorite consumption; and near‐simultaneous development of andalusite + K‐feldspar and sillimanite, the latter indicating a pressure of contact metamorphism of ~3 kbar. In other parts of the aureole, the development of sillimanite downgrade of the initial development of K‐feldspar suggests slightly higher pressures of contact metamorphism. Lack of correspondence between the observed sequence of reactions in the aureole and those predicted thermodynamically suggests that modifications to some of the thermodynamic data or activity–composition models may be required. Textural features in the aureole suggest the influence of kinetic factors on metamorphic recrystallization, including: (i) deformation‐catalysed reaction in the schists compared to the hornfelses, as indicated by different mineral‐growth sequences inferred from microstructures, and (ii) heating rate‐controlled recrystallization, as indicated by the decrease in grain size of hornfelses with increasing metamorphic grade.     

Stepping stones and pitfalls in the determination of an anticlockwise P-T-t-deformation path: the low-P, high-T Cooma Complex, Australia

Abstract Low-pressure/high-temperature (low-P/high-T) metamorphic rocks of the Cooma Complex, southeastern Australia, show evidence of an anticlockwise pressure-temperature-time-deformation (P-T-t-D) path, similar to those of some other low-P/high-T metamorphic areas of Australia. Prograde paths are reasonably well constrained in cordierite-andalusite schists, cordierite-K-feldspar gneisses and andalusite-K-feldspar gneisses. These paths are inferred to be convex to the temperature axis, involving increase in pressure with increase in temperature. Evidence of the retrograde path is inconclusive, but is consistent with approximately isobaric cooling, as are available isotopic data on the Cooma Granodiorite, which indicate initially rapid cooling following attainment of peak temperatures. The retrograde path is inconsistent with either a clockwise P-T-t-D path involving rapid or even moderate decompression immediately post-dating the peak of metamorphism, or a path in which the retrograde component simply reverses the prograde component, because both these paths should cross reactions forming cordierite from aluminosilicate, for which no evidence has been observed. Determination of the deformational-metamorphic history of the complex is not straightfoward and depends on careful examination of critical samples. Evidence necessary for successful elucidation of the prograde, and part of the retrograde, deformational-metamorphic history in the Cooma Complex includes: (1) sequentially grown porphyroblasts that can be timed relative to surrounding foliations; (2) partial replacement microstructures providing relative timing of metamorphic reactions that cannot be timed relative to foliation development; (3) a tectonic marker foliation (S4 at Cooma) that allows correlation of foliations from one location to another; and (4) single samples containing all of the foliations and all generations of porphyroblast growth within a single metamorphic zone. The latest two or three foliations involve low strain accumulation, allowing relative timing relationships between foliations and porphyroblasts to be more clearly determined. Sequential porphyroblast growth and foliation development in the cordierite-andalusite schists is examined for situations involving rotation and non-rotation of porphyroblasts relative to geographically fixed coordinates. Although the number of foliations developed varies in the rotational situation, depending on the deformation history proposed, the sequential order of porphyroblast growths does not differ from the non-rotational situation. Thus, whether or not porphyroblasts rotated in the Cooma rocks, the sequence of reactions, and therefore P-T-t paths inferred from the relative timing of porphyroblast growths, remain the same, for the deformational histories evaluated.