Basaltic Amphibolites Research Articles

Tracing proto-Rheic - Qaidam Ocean vestiges into the Western Tatra Mountains and implications for the Palaeozoic palaeogeography of Central Europe

Zircon petrochronology from amphibolites and retrogressed eclogites from the basement of the Western Tatra Mountains (Central Western Carpathians) reveals a complex rock evolution. An island-arc related basaltic amphibolite from Žiarska Valley shows three distinct zircon forming events: igneous zircon growth at ca. 498 Ma (Middle/Late Cambrian) and two phases of amphibolite-facies metamorphism at ca. 470 Ma (Early Ordovician) and at ca. 344 Ma (Early Carboniferous). A retrogressed eclogite from Baranèc Mountain records two zircon forming events: metamorphic zircon growth under eclogite-facies conditions at ca. 367 Ma (Late Devonian) and amphibolite-facies metamorphism at ca. 349 Ma (Early Carboniferous). These data contribute towards understanding and correlating major tectonothermal events that shaped the eastern margin of Gondwana in the Early Palaeozoic and its subsequent Variscan evolution. The metabasites record vestiges of two completely independent oceanic domains preserved within the Central Western Carpathians: (1) An Ediacaran to Cambrian oceanic arc related to the proto-Rheic - Qaidam oceans and metamorphosed to amphibolite-facies in the Early Ordovician subduction of the proto-Rheic - Qaidam arc during the Cenerian orogeny (ca. 470 Ma) and (2) Late Devonian oceanic crust related to a back-arc basin (Pernek-type), formed by the opening of the Paleotethys and metamorphosed to eclogite-facies during Devonian subduction (ca. 367 Ma). The common Variscan and later evolution of these oceanic remnants commenced with amphibolite-facies metamorphic overprinting in the Early Carboniferous (amphibolite: ca. 344 Ma; retrogressed eclogite: ca. 349 Ma) related to an Early Variscan consolidation and the formation of Pangea. None of the investigated rocks of the Central Western Carpathians show any evidence of being chronologically or palaeogeographically related to the Rheic Ocean, therefore any prolongation of the Rheic suture from the Sudetes into the Alpine-Carpathian realm is highly problematic. Instead, the Southern and Central Alpine Cenerian orogeny can be traced into the Central Western Carpathians.

Banded amphibolites in the Alps: a new interpretation in relation to early Paleozoic peraluminous magmatism

The Strona-Ceneri Zone is located south of the Insubric line, where Alpine overprint is mainly brittle and of low grade or even absent. Apart from the unmetamorphic Permo-Carboniferous sediments and intrusiva the Strona-Ceneri Zone is an Ordovician gneiss complex composed of paragneisses, migmatites, peraluminous orthogneisses and banded amphibolites associated with meta-gabbros and meta-ultramafics. Despite of the individual characters of the other Ordovician gneiss terranes north of the Insubric line, most prominently the Aar Massif, the Gotthard, Silvretta and Ötztal nappes, they show analogous lithological formations and pre-Mesozoic structures indicating similar genetic processes. A revision of geological maps and new field observations in these gneiss terranes indicate characteristic spatial relationships of banded amphibolite formations with migmatites and orthogneisses. The contrasting chemistries of the peraluminous rocks (para- and orthogneisses) and the basaltic amphibolites with a lack of intermediate lithologies in between can be explained by the setting of peraluminous arc magmatism within a subduction–accretion complex. In this model the amphibolites represent primary basalts which ponded at the base of the subduction–accretion complex and delivered the heat for the production of peraluminous melts. Volume estimations indicate a “zone of intermingling” composed of immiscible basalts and peraluminous melts with a total thickness of several kilometers. In this “zone of intermingling” the protoliths of the banded amphibolite formations were generated. Steep strike-slip faults, which are important structures for the syn-magmatic cratonization of subduction–accretion complexes, provide pathways for the emplacement of magmas, migmatites and intermingled materials. This results in the formation of steeply oriented sheets of orthogneisses, deformed migmatites and banded amphibolite formations, respectively. Finally, the paper lists many other peri-Gondwanan regions with an early Paleozoic peraluminous arc magmatism, indicating a similar setting on a global scale.

A Neoarchean arc-backarc pair in the Linshan Massif, southern North China Craton

The Linshan Massif is located in the southern North China Craton (NCC) and consists mainly of tonalitic (TTG) and dioritic gneisses, and a volcano-sedimentary-dominated assemblage, all of which were intruded by post-collisional potassic granites. The volcano-sedimentary-dominated assemblage is composed mostly of deformed and foliated basaltic amphibolite and meta-sedimentary rocks, with minor interlaying meta-gabbros, forming a stack of tectonically imbricated sedimentary and volcanic rocks. The TTG gneisses formed at 2.6–2.52 Ga, and the meta-gabbros, basaltic amphibolites and diorites formed at 2.51–2.50 Ga. They underwent subsequent metamorphism at 2.49–2.48 Ga. Furthermore, all their igneous zircons possess large positive εHf (t) values (+2.9 to +7.4). The basaltic amphibolites can be divided into two groups: depleted basalts and enriched basalts. Depleted basalts are characterized by flat LREE patterns and low Th/Nb ratios with positive Nb anomalies, similar to N-MORB-type patterns. Enriched basalts display LREE-enriched patterns with high Th/Nb ratios and pronounced negative Nb anomalies, showing an arc-related affinity. The meta-gabbros have LREE-enriched patterns with negative Nb and Zr anomalies, consistent with subduction-related signatures. Considering mixed arc- and MORB- signatures with positive zircon εHf(t) values, we propose that the meta-gabbros and basaltic amphibolites may have formed in a back-arc setting. The tonalites and the diorites showing LREE-enriched patterns and depleted in HFSEs are interpreted to represent arc-related magmatism. Combined with the documented 2.55 Ga Dengfeng fore-arc accretionary complex, we propose the following geodynamic evolution model for the Southern NCC: (1) at 2.6–2.54 Ga, a TTG-dominated oceanic arc developed above a west-dipping subduction zone, forming the 2.55 Ga Dengfeng accretionary complex and related tectonic units; (2) at 2.52–2.50 Ga, slab rollback subsequently drove back-arc rifting of the mature TTG-dominated intra-oceanic arc, resulting in the formation of two arc segments with the intervening Linshan back-arc basin; and (3) at 2.50–2.48 Ga, the TTG-dominated arc collided with the Eastern Block resulting in the closure of the back-arc basin and formation of the accreted arc-back-arc package in the Linshan Massif. We suggest that the Neoarchean Central Orogenic Belt in the southern NCC contains at least two amalgamated oceanic arc terranes, separated by a belt of back-arc basin magmatic rocks caught within an intra-oceanic mélange/suture zone.

A 2.5 Ga fore-arc subduction-accretion complex in the Dengfeng Granite-Greenstone Belt, Southern North China Craton

The Dengfeng granite-greenstone belt (DGGB), located in the southern segment of the Central Orogenic Belt (COB) of the North China Craton (NCC), consists of a volcano-sedimentary assemblage, intruded by tonalite, trondhjemite, granodiorite (TTG suite), diorite, granites and late mafic dikes. The volcano-sedimentary assemblage in the DGGB mainly consists of tectonically imbricated basaltic amphibolites, meta-gabbroic rocks with minor ultramafic rocks, and metagreywacke, marble and quartzite, consistent with characterisitics of typical Phanerozoic subduction-accretion complexes. The basaltic amphibolites yield a metamorphic zircon 207Pb/206Pb age of 2507±26 Ma, interpreted to represent the peak age of amphibolite facies metamorphism that took place during subduction/accretion of the basaltic protolith. The basaltic amphibolites are characterized by a tholeiitic affinity, and flat LREE patterns with minor negative Nb and Zr anomalies. Based on mixed MORB- and arc-affinities, the basaltic amphibolites in the DGGB are interpreted to have formed in a fore-arc tectonic setting. One late potassic granite dike cutting across the fabrics of the volcano-sedimentary assemblage yields an intrusion age of 2492±35 Ma, constraining the minimum deformation age for tectonic assembly of the package. The TTG gneisses and diorites intruded the western margin and the center of the subduction-accretion complex, respectively. One TTG sample and one diorite sample yield igneous zircon 207Pb/206Pb ages of 2514±26 Ma and 2518±36 Ma, respectively, constraining their intrusion ages. The TTG gneisses display high ratios of (La/Yb)cn and Sr/Y, and depletion in HFSE with negative Nb, Ta and Ti anomalies, consistent with those of typical Archean TTGs. The TTG gneisses are therefore considered to be generated from partial melting of a shallowly subducting oceanic slab and/or accreted arc amphibolites. The diorites have high concentrations of MgO (2.89–6.05wt.%), Ni (148–178ppm) and Cr (85.7–120.6ppm), and highly fractionated REE patterns and are depleted in HFSEs with negative Nb, Ta, Zr, Hf and Ti anomalies. We suggest that the high-Mg diorites in the DGGB may have been derived from a hydrated mantle wedge which was previously metasomatized by subduction-derived melts and/or fluids. Collectively, a Neoarchean subduction-accretion-collision event is therefore proposed to have generated the DGGB. The volcano-sedimentary assemblage in the DGGB represents a fore-arc subduction-accretion complex, which is interpreted to be related to the suture zone of ca. 2.5 Ga arc-continent collision between a TTG-dominated arc terrane in the COB and the Eastern Block of the NCC. We further propose a long N-S striking Neoarchean suture zone occurring in the eastern margin of the COB mainly consists of the ca. 2.5 Ga subduction-accretion complex in the DGGB to the south, the ca. 2.5 Ga mélange belt in the Zanhuang Complex in the central, and the ca. 2.5 Ga ophiolitic mélange belt in the Zunhua-Dongwanzi structural belt to the north, which separate an arc terrane in the COB from the Eastern Block of the NCC.

Fluid flux melting generated postcollisional high Sr/Y copper ore–forming water-rich magmas in Tibet

Miocene postcollisional porphyry Cu deposits in southern Tibet are genetically associated with dacitic-rhyolitic intrusions with unusually high Sr/Y ratios (>40), which have been attributed to dehydration melting of garnet amphibolite in a thickened lower crust. To test this hypothesis and examine the hydration state of copper ore-forming high Sr/Y magmas, we utilize a geohygrometer for granitoid rocks, entailing zircon-saturation thermometry and H 2 O-dependent phase equilibria. The results show that these Tibetan high Sr/Y magmas had dissolved H 2 O contents >10 wt%, which considerably exceeds the water supply by dehydration melting of basaltic amphibolites (maximum of 6.7 ± 1.4 wt%). Our results indicate that high Sr/Y dacitic-rhyolitic magmas cannot be produced by dehydration melting of basaltic amphibolites. While H 2 O-added melting of basaltic amphibolites can produce high Sr/Y dacitic-rhyolitic melts, it does not yield high enough Mg# (>50) to match the Tibetan ore-forming porphyries. We propose an alternative model for the genesis of copper ore-forming high Sr/Y magmas in Tibet, and suggest that the high Sr/Y dacitic-rhyolitic porphyries in southern Tibet are residually H 2 O-enriched, high-pressure differentiation products of hydrous mafic partial melts of Tibetan mantle. This hypothesis is based on the previous investigation of Miocene mafic microgranular enclaves (mantle-derived melts), which define a fractionation trend with, and have Sr-Nd-Hf isotopic compositions similar to, the host Tibetan ore-forming porphyries.

The Chimalpahad anorthosite Complex and associated basaltic amphibolites, Nellore Schist Belt, India: Magma chamber and roof of a Proterozoic island arc

New major and trace element data on the Proterozoic Chimalpahad layered anorthositic Complex and associated basaltic amphibolites of the Nellore Schist Belt of South India provide new constraints on their petrogenesis and geodynamic setting. The Complex consists of layered anorthosites, leucogabbros, gabbros, ultramafic rocks and is spatially associated with basaltic amphibolites. Despite deformation and metamorphism, primary cumulate textures and igneous layering are locally well preserved throughout the Complex. Whereas the amphibolites display diverse REE systematics, the Chimalpahad anorthositic–gabbroic rocks are characterized by moderately depleted to strongly enriched LREE patterns and by flat to depleted HREE patterns. The field relations, major and trace element compositions of the basaltic amphibolites suggest that they are petrogenetically related to the anorthositic–gabbroic rocks by fractional crystallization. The anorthositic rocks and the basaltic amphibolites share the depletion of Nb relative to Th and La on primitive mantle-normalized diagrams. They exhibit signatures of arc magmatic rocks, such as high LILE and LREE relative to the HFSE and HREE, as well as high Ba/Nb, Ba/Zr, Sr/Y, La/Yb ratios that mimic chondrite-normalized REE and primitive mantle-normalized trace element patterns of arc magmas. Similarly, on log-transformed tectonic discrimination diagrams, the Chimalpahad rocks plot within the field of Phanerozoic magmatic arcs, consistent with a subduction zone origin. On the basis of field relations and geochemical characteristics, the Chimalpahad Complex is interpreted as a fragment of a magma chamber of an island arc, which is tectonically juxtaposed against its original volcanic cover. A new preliminary Sm–Nd date of anorthosite from the Chimalpahad Complex indicates a model age of 1170 Ma.

Archean crustal evolution of the Aldan Shield, Siberia: geochemical and isotopic constraints

The relatively unknown Aldan Shield in eastern Siberia is made up of Archean granite–greenstone and high-grade gneiss terrains as commonly observed in many Precambrian shield areas. New results of geochemical and isotopic study on two terrains of contrasting metamorphic grades (Olekma Granite–Greenstone and West Aldan Granulite Gneiss terrains) are present with the following principal conclusions being reached: (1) the crustal evolution of the Aldan Shield started as early as 3.5 Ga as suggested by the T DM model ages of a few high-grade gneisses and a retrograded granulite enclave, but the most important crust-forming and tectonothermal event is firmly established at ca 3.0 Ga. (2) New SHRIMP and Pb evaporation analyses on zircons have confirmed the presence of crustal rocks older than 3 Ga. However, the zircon U–Pb systems appear to have been disturbed by later thermal effects, and no components of ≥3.5 Ga have been identified. (3) Komatiitic and basaltic amphibolites form important supracrustal sequences in the Olekma terrain. Both Group I and II komatiites occur and garnet (or majorite) fractionation in mantle melting processes is likely to have taken place during the genesis of group II komatiites. Overall, depleted mantle sources were involved in the geneses of basic and ultrabasic magmas as evidenced from LREE depletion and Nd isotopic compositions [ ϵ Nd( T)=+2]. Crustal contamination was not significant as suggested by the low La/Nb ratios (=1) in komatiites and metabasalts. (4) As common to most Archean tonalite–trondhjemite–granodiorite (TTG) rocks, the Aldan granitic gneisses also show highly fractionated rare earth element (REE) patterns with heavy REE depletions, suggesting that separation of garnet and amphibole has occurred during melting of their mafic sources. (5) Although the Olekma and West Aldan terrains have undergone different metamorphic evolution, there is no substantial difference in the time of major crust-forming events at ∼3.0 Ga and a strong Proterozoic thermal disturbance at ∼2 Ga. Furthermore, from the geochemical comparison of the two terrains it is concluded that the nature of mantle sources for basic–ultrabasic magmas and the process of TTG magma generation are similar in both terrains and that the granulite facies rocks cannot be considered as the restites of intracrustal melting.

Metamorphosed and dismembered ophiolite suites in the basement units of the Eastern Alps

The pre-Alpine basement of the Eastern Alps contains several ultramafic/mafic rock complexes, four of which have been investigated. Although polyphase deformation and metamorphism obscures much of the primary features, field, petrographical and geochemical evidence favours ophiolitic origins in mid-ocean ridge and back-arc tectonic settings. The Stubach Complex belongs to the Penninic basement of the Tauern Window and is interpreted as an obducted back-arc ophiolite with ultramafic cumulates, gabbro, and minor amounts of basalt, which shows the geochemistry of subduction-related magma generation. The Rating Complex belongs to a group of strongly dismembered basement slices carrying the Palaeozoic Austroalpine sequence of the Northern Greywacke Zone. Basaltic rocks are accompanied by former metamorphic peridotite containing chromite concentrations. Nearly all members of an ophiolite suite are present in the Speik Complex in the Austro-Alpine crystalline basement. The source rocks are metamorphic peridotite (harzburgite) with chromite occurrences, cumulate ultramafic and mafic rocks, isotropic gabbro, basalt and/or dolerite, and oceanic sediments. The Plankogel Complex, also part of the Austro-Alpine crystalline basement, represents a tectonic mélange. Blocks of serpentinite and amphibolite derived from metamorphic peridotites and mid-ocean ridge basalts, together with marble and quartzite, are contained in a micaschist matrix. Admixed is a rock assemblage with alkali basaltic amphibolites, interpreted as a seamount environment. The Ritting, Plankogel, and possibly parts of the Speik Complexes contain relict high-pressure minerals and therefore went through a stage of subduction. The inferred ages of all complexes is Late Precambrian to Early Palaeozoic. The ophiolite suites have their distinct characteristics and contrasting tectonic settings, and are therefore considered to be genetically unrelated to each other.

Origin of late Archean granite: geochemical evidence from the Vermilion Granitic Complex of northern Minnesota

The 2,700-Ma Vermilion Granitic Complex of northern Minnesota is a granite-migmatite terrane composed of supracrustal metasedimentary rocks, mafic rocks, tonalitic and granodioritic plutonic rocks, and granite. The metasedimentary rocks are predominantly graywacke, which has been regionally metamorphosed to garnet-sillimanite-muscovite-bearing biotite schist, and has locally undergone anatexis. The mafic rocks form early phases within the complex and are of two types: (1) basaltic amphibolite, and (2) monzodiorite and essexite rich in large ion lithophile elements (LILE). The members of the early plutonic suite form small bodies that intrude the metasedimentary rocks and mafic rocks, producing an early migmatite. The granite is of two distinct varieties: (1) white garnet-muscovite-biotite leucogranite (S-type; Chappell and White 1974) and (2) grayish-pink biotite-magnetite Lac La Croix Granite (I-type). The leucogranite occurs in the early migmatite and in paragneissic portions of the complex, whereas the Lac La Croix Granite is a late-stage intrusive phase that invades the early migmatite and metasediment (producing a late migmatite) and forms a batholith. This study focuses specifically on the origin of granite in the Vermilion Granitic Complex. Chemical mass-balance calculations suggest that the S-type two-mica leucogranite had a metagraywacke source, and that the I-type Lac La Croix Granite formed via partial fusion of calc-alkaline tonalitic material, which may have been similar to rocks of the early plutonic suite. This model is satisfactory for petrogenesis of similar Late Archean post-kinematic granites throughout the Canadian Shield.

The Entire anorthositic gneiss, eastern Arunta Inlier, Central Australia: Geochemistry and petrogenesis

A cumulus igneous origin is proposed for anorthositic gneisses, anorthositic metagabbros and meta‐ultramafic rocks comprising the Entire anorthositic gneiss member of the Early Proterozoic Harts Range meta‐igneous complex (HRMC), eastern Arunta Inlier. The ultramafic and anorthositic gneisses possess distinctive whole‐rock chemistries which are the result of cumulus components with widely different proportions of variably evolved intercumulus liquid. A comagmatic relationship between these rocks and their host HRMC amphibolites is supported by a strong similarity between the observed trace element chemistry of the amphibolites and the trace element compositions of calculated equilibrium liquids from which the cumulates crystallized. In addition, some garnet‐bearing anorthositic metagabbros have absolute trace element abundances similar to the amphibolites. The parental magma for the ultramafic‐anorthositic rocks was a depleted aluminous tholeiitic melt selectively re‐enriched in large ion lithophile (LIL) elements and with lower Mg/(Mg + Fe2+) ratios and Ni contents than expected in primary melts, due to early olivine‐controlled fractionation. Extensive plagioclase+clinopyroxene‐controlled low‐pressure fractionation of the evolved melt abstracted bulk mineral assemblages comprising the anorthositic rocks and yielded the tholeiitic fractionation trend of the host basaltic amphibolites. Periodic removal of olivine+pyroxene‐dominated assemblages gave rise to the ultramafic cumulates and also contributed to the concomitant iron enrichment trend in residual liquids. The Entire anorthositic and ultramafic gneisses show mineralogical and geochemical features more closely analogous to Archaean stratiform anorthosite complexes than to Mid‐Proterozoic massif‐type anorthosite bodies. The suite is interpreted as the metamorphosed remnant of an extensive layered stratiform complex. Intrusion probably took place in a rifting continental margin environment where thin continental crust facilitated rapid ascent of the mantle‐derived tholeiitic magma to a high level, permitting fast but limited differentiation to yield the layered sequence.

Banded amphibolites of the harts range meta-igneous complex, central Australia: an early proterozoic basalt-tonalite suite

A multistage petrogenesis is proposed for banded metabasaltic amphibolite and tonalite tectonically interlayered on a fine (centimetre) scale within upper and lower amphibolite sequences of the Harts Range meta-igneous complex, eastern Arunta Block, central Australia. The basaltic amphibolites exhibit only slight rare earth element (REE) fractionation ( Ce N Yb N = 2.5 ) and can be related by plagioclase + clinopyroxene — dominated low-pressure fractional crystallisation in the tholeiitic trend. Early olivine removal from a moderately aluminous, trace element impoverished tholeiitic picrite parent magma caused significant Ni depletion in the evolved residual liquids that gave rise to the basaltic amphibolites. The primary picritic melt was generated by high degrees (> 30%) of hydrous partial melting of a depleted mantle source selectively re-enriched in large-ion lithophile (LIL) elements. Chondrite normalised REE patterns for low-Al 2O 3 tonalitic layers within the basaltic amphibolite units are moderately fractionated ( Ce N Yb N = 3.3–7.3 ) and show significant negative Eu anomalies. REE modelling indicates that the precursors to the tonalitic rocks could have been generated by small degrees (≈ 10%) of partial melting of a light rare earth element (LREE) enriched basaltic parent with plagioclase, clinopyroxene and minor hornblende comprising coexisting residual phases. Melting at relatively low P H 2O (< 0.5 kbar) and depths between 20 and 30 km is indicated. The amphibolite sequence is interpreted as a complexly folded and metamorphosed bimodal basalt-tonalite suite formed at an early Proterozoic rifted continental margin or intra-cratonic rift.

Geochemistry and petrogenesis of Archaean metavolcanic amphibolites from Fiskenaesset, S. W. Greenland

The meta-volcanic amphibolites closely associated with the Fiskenaesset anorthosite complex can be subdivided on the basis of trace element patterns or mineral chemistry; by far the most abundant type has light rare-earth element (LREE) depleted REE patterns and displays a wide range in trace element abundances. Chemically comparable amphibolites can be recognised throughout the ca. 2800 M.yr. West Greenland terrain. The geochemistry of the basaltic amphibolites is dominantly controlled by fractional crystallisation processes, although variable degrees of partial melting may also be important. The required crystal extract (plagioclase dominated) in the proposed fractionation scheme is very similar to the primary mineralogy of cumulates of the Fiskenaesset complex and trace element models support a genetic relationship between the anorthosite complex and enclosing host amphibolites. The application of trace element discrimination to assign tectonic environment in the Archaean is arguable. However, details of the trace element chemistry (especially chondritic La/Ta ratios) are taken to suggest, out of a range of likely tectonic environments, an ocean floor, rather than island arc, affinity for the Fiskenaesset amphibolites. The large ion lithophile (LIL) elements display erratic distribution patterns, but are generally enriched relative to the REE. This appears not to be related to high-grade metamorphism but may be a relict feature of seafloor alteration. The association of the cumulate sequence with meta-volcanic amphibolites and metasediments probably represents an ocean floor assemblage emplaced into the lower crust during crustal accretion.

Isotopic studies on a Precambrian geochronologic boundary, Sierra Madre Mountains, Wyoming

Geochronologic evidence indicates that a profound discontinuity separating two discrete Precambrian provinces parallels the Wyoming-Colorado border. This east-trending boundary transects the Sierra Madre Range in southern Wyoming adjacent to a fault-bounded sequence of folded middle Precambrian sedimentary rocks. Precambrian rocks of the Sierra Madre Mountains reveal a series of igneous and sedimentary events that occurred near the probable edge of the Archean continental nucleus between 2,600 and 1,600 m.y. B.P. North of the boundary, Archean quartz-biotite gneiss, probably 2,800 to 2,900 m.y. old, grades eastward into a northwest-trending antiform of microcline augen gneiss and granite. The Rb-Sr isochron of the granite gneiss complex yields an age of 2,560 m.y. and an initial Sr 87 /Sr 86 intercept of 0.7000. Geochemical evidence suggests that the quartz-biotite gneiss may have evolved from a supracrustal dacitic protolith formed by anatexis of a basaltic amphibolite. Calc-alkalic volcanic flows, tuffs, and volcanogenic sediments and a 4-km-thick sequence of conglomerate, quartzite, and limestone were deposited on the gneiss basement after 2,560 m.y. B.P. A period of igneous activity and tectonism between 1,650 and 1,900 m.y. B.P. resulted in the metamorphism and folding of the sediments into an east-trending anticlinorium of isoclinal folds. The Sierra Madre south of this metasedimentary structure consists principally of a composite batholith of plutons ranging from gabbro to granite. The Encampment River Granodiorite, the oldest intrusion, is a gneissic unit that was probably derived by partial melting and remobilization of the polycyclic quartz-biotite gneiss, as suggested by isotopic and rare-earth-element data. This thermal reactivation occurred between 1,900 and 2,000 m.y. B.P. The majority of the batholith is made up of a distinctive red granite that yields an isochron age of 1,680 m.y. and an initial Sr 87 /Sr 86 ratio of 0.7024. A white quartz monzonite discordantly intrudes the granite and hornblende gneiss country rock. Isochron ages and initial Sr 87 /Sr 86 intercepts of the monzonite and gneiss are 1,680 m.y. and 0.7020 for the quartz monzonite and 1,880 m.y. and 0.7015 for the country rock. At least three distinct generations of mafic dikes are present in the Sierra Madre; field relations with dated units indicate ages of ∼2700 m.y., 1,800 to 2,650 m.y., and ∼1,680 m.y.

Origin of trondhjemitic biotite—quartz—oligoclase gneisses from the Venezuelan Guyana Shield

Biotite—quartz—oligoclase gneisses constitute the dominant lithology in a 3-b.y.-old metamorphic assemblage in the Venezuelan Guyana Shield. The assemblage includes basaltic amphibolites as well as granitic gneisses, iron formation, and other metasedimentary lithologies. Consideration of major- and trace-element compositions indicates that both the biotite gneisses and basaltic amphibolites are meta-igneous. The amphibolites have oceanic tholeiite compositions. Two groups of biotite gneisses can be differentiated by chemical criteria. Both groups have major and trace-element compositions which allow their derivation as partial melts of tholeiite compositions at mantle depth, as has been suggested for similar rocks in other areas. However, the compositional correspondence of the gneisses with low variance liquidus loci in low- P T synthetic systems, and their relatively oxidized character, appear more compatible with an origin by partial melting of graywackes at crustal levels. For both groups of gneisses, primary melts with relatively low Na 2O K 2O ratios can be postulated which are credibly derived by melting of graywackes; reasonable fractionation processes can be hypothesized to explain the compositional variations within each group. Although stratigraphic relations in the Venezuelan Guyana Shield are uncertain, geologic relations in and near the map area allow, as valid working hypotheses, stratigraphic sequences which parallel those which have been recognized in better-known Archean terranes.

Experimental study of amphibolite and eclogite stability

The amphibolite-eclogite transition is often observed in metamorphic rocks and may be expected to occur in the lower crust or upper mantle if P H 2O ≈ P S. Many simple amphiboles become unstable at high water pressures due to backbending of dehydration reactions or to solid-solid transitions involving sheet silicates. Contrary to Yoder and Tilley (1962), eclogite is expected to be stable relative to amphibolite at sufficiently high water pressures because of the formation of dense garnet and the high compressibility of water vapor. Experiments in progress on several basaltic compositions have produced eclogite at the expense of amphibolite at high water pressures. Seeding has been found to be important as hornblende crystallizing from glass will persist at least 4 kb above its stability field defined by reversals with garnet-pyroxene-amphibole mixtures. In reconnaissance experiments on a quartz tholeiite and an alkali olivine basalt, hornblende disappears at high water pressures between 15–25 kb and 700–900°C. Biotite appears to be stable in the alkali olivine basalt to at least 40 kb at these temperatures. Basaltic amphibolites do not appear to be stable very far into the low-velocity zone even at P H 2O = P S confirming Lambert and Wyllie's (1968) earlier experiments. Biotite however seems to be stable deeper in the mantle (at least for potassic basalts) than previously considered possible if P H 2O ≈ P S and may be a source of water for magmas when partially melted.