Trondhjemitic Magmas Research Articles

Adakitic high-Al trondhjemites in the Proterozoic Østfold-Marstrand Belt, W Sweden

This paper investigates the first identified intrusives in SE Norway–W Sweden with the specific signature of adakitic arc magmas, which in recent settings are preferably explained as partial melts extracted from subducted oceanic crust. The studied adakitic high–Al trondhjemites occur as sheets in the Koster archipelago, W Sweden, where they form the oldest recognized granitoids in the metasupracrustals of the Stora Le–Marstrand formation. The trondhjemites were intruded during a short ca. 1.59–1.58 Ga interlude between the early and the main orogenic events of the Gothian orogeny (1.6–1.56 Ga, Åhäll et al. 1998). This interlude is otherwise characterized by ‘ordinary’ calcalkaline magmatism which on Koster is predated by the trondhjemites. The typical adakitic signature suggests that the trondhjemitic magma was extracted from a MORB (Mid Ocean Ridge Basalt) like source, and that a hornblende eclogite restite was left in the region of melting. The restite composition indicates melt extraction at PT conditions in the range of 18–25 kb/800°C to 13-15 kb/950–1050°C. These requirement can only be met by subduction of warm (young or shear heated) oceanic crust beneath a crust including early Gothian metamorphosed and deformed Stora Le–Marstrand formation or by melting of metabasaltic material at a deep crustal level. The latter is a less likely possibility and demands that the Stora Le–Marstrand formation at the time of melt extraction was part of a > 45 km thick crust.

Source and tectonic implications of tonalite-trondhjemite magmatism in the Klamath Mountains

In the Klamath Mountains, voluminous tonalite-trondhjemite magmatism was characteristic of a short period of time from about 144 to 136 Ma (Early Cretaceous). It occurred about 5 to 10 m.y. after the ∼165 to 159 Ma Josephine ophiolite was thrust beneath older parts of the province during the Nevadan orogeny (thrusting from ∼155 to 148 Ma). The magmatism also corresponds to a period of slow or no subduction. Most of the plutons crop out in the south-central Klamath Mountains in California, but one occurs in Oregon at the northern end of the province. Compositionally extended members of the suite consist of precursor gabbroic to dioritic rocks followed by later, more voluminous tonalitic and trondhjemitic intrusions. Most plutons consist almost entirely of tonalite and trondhjemite. Poorly-defined concentric zoning is common. Tonalitic rocks are typically of the low-Al type but trondhjemites are generally of the high-Al type, even those that occur in the same pluton as low-Al tonalite. The suite is characterized by low abundances of K2O, Rb, Zr, and heavy rare earth elements. Sr contents are generally moderate (∼450 ppm) by comparison with Sr-rich arc lavas interpreted to be slab melts (up to 2000 ppm). Initial 87Sr/86Sr, δ18O, and ɛNd are typical of mantle-derived magmas or of crustally-derived magmas with a metabasic source. Compositional variation within plutons can be modeled by variable degrees of partial melting of a heterogeneous metabasaltic source (transitional mid-ocean ridge to island arc basalt), but not by fractional crystallyzation of a basaltic parent. Melting models require a residual assemblage of clinopyroxene+garnet±plagioclase±amphibole; residual plagioclase suggests a deep crustal origin rather than melting of a subducted slab. Such models are consistent with the metabasic part of the Josephine ophiolite as the source. Because the Josephine ophiolite was at low T during Nevadan thrusting, an external heat source was probably necessary to achieve significant degrees of melting; heat was probably extracted from mantle-derived basaltic melts, which were parental to the mafic precursors of the tonalite-trondhjemite suite. Thus, under appropriate tectonic and thermal conditions, heterogeneous mafic crustal rocks can melt to form both low- and high-Al tonalitic and trondhjemitic magmas; slab melting is not necessary.

Trace elements in diamond inclusions from eclogites reveal link to Archean granites

We report trace element data for rare garnet and clinopyroxene inclusions trapped within diamonds occurring in eclogite xenoliths from the Siberian craton. These inclusions are more depleted in incompatible trace elements and have lower Mg# than the eclogite host minerals, reflecting metasomatic enrichment of the eclogites after diamond formation by high Mg# melts that are enriched in incompatible trace elements (such as kimberlites). A reconstructed whole rock composition (using inclusion data) is severely depleted in light rare earth elements and niobium but shows a marked strontium enrichment. The melt in equilibrium with this eclogite is calculated to have had very high La Yb and Sr Nd and is Nb depleted (if rutile is present), characteristics similar to those of some convergent margin magmas but most especially Archean tonalitic rocks. Experimental petrologic studies demonstrate that eclogites of the type described here are in equilibrium with silicic melts such as tonalites or trondhjemites. Thus, both our trace element data and petrological considerations are consistent with the eclogites forming in equilibrium with Archean tonalitic or trondhjemitic magmas. Our data may therefore comprise the first evidence for the fate of these voluminous residues. A corollary of our findings is that trace element and isotopic data from eclogite minerals must be interpreted with caution because of the eclogite's complex evolutionary history. Specifically, models of early Earth differentiation based on isotopic data from Siberian eclogites [1,2] require re-evaluation.

Reassessment of Archean crustal development in the Barberton Mountain Land, South Africa, based on U‐Pb dating

This paper presents U‐Pb ages for 23 samples from the Barberton Mountain Land, an Archean granite‐greenstone terrain in the Kaapvaal craton of South Africa. A reexamination of the geology of this area in the light of previous U‐Pb dating and the new ages indicates that much of its deformation can probably be accounted for by a single short‐lived compressional event within the time span 3230–3215 Ma which was synchronous with deposition of the Moodies Group and the upper part of the Fig Tree Group sediments. This event may have been responsible for thrusting of much older sequences in the Onverwacht Group, including slivers of 3.54 Ga old gneiss, circa 3.47 Ga old komatiitic and tholeiitic sequences, and circa 3.45 Ga old felsic sequences. Key constraints on the timing of deformation are previously determined 3227–3225 Ma ages for detrital zircons in thrusted units of the Fig Tree Group; a 3229 +4/−3 Ma age for a deformed porphyry which is pre‐ or synthrusting; a previously determined 3227 ± 3 Ma age for an undeformed porphyry in an inferred thrust fault; 3227 ± 1 Ma ages for both the Kaap Valley pluton and an ignimbrite in the Stolzburg syncline, both of which predate regional upright folding; and a 3216 ± 2 Ma age for the Dalmein pluton which appears to cut regional, upright NE‐SW trending folds in the Kromberg syncline, part of the final phase of deformation associated with regional compression. Areally extensive granitoid sheets and syenogranitic plutons were subsequently emplaced into the Barberton region at about 3105 Ma. The linear configuration of the syenogranitic plutons, which are also aligned with the gabbroic Usushwana complex as well as the geometry of the granitoid sheets, suggests that the circa 3105 Ma magmatism was caused by incipient rifting around a NW‐SE trending crustal fracture. This episode was probably coeval with a late regional transtensional phase of deformation recorded in the greenstone belt. Some of the above interpretations are in conflict with previous models for early development of the Barberton greenstone belt. A 3352 ± 6 Ma age from a metagabbro which is comagmatic with a proposed ultramafic sheeted dike complex is over 100 m.y. younger than the probable age of volcanic rocks which host the dikes. This conflicts with the suggestion that much of the Onverwacht Group comprises an ophiolitic section, obducted in an intraoceanic setting at circa 3.45 Ga, that partially melted to form trondhjemitic magmas which intruded along active thrust faults. Present data suggest that, despite its antiquity and the diversity of ages and lithologies found within it, the Barberton greenstone belt can generally be understood in terms of actualistic tectonic processes at convergent margins.

Experimental melting of hydrous low-K tholeiite: evidence on. the origin of Archaean era tons

Experimental melting studies were performed on a natural high-Al basalt and a synthetic average Archaean tholeiite (AAT) composition (0.3 wt.% K20) with variable amounts of H20. Microprobe analyses of quenched melts (glass) from runs at 5-30 kbar and 750°-l100°C showed that typical Archaean tonalitic and trondhjemitic "grey gneiss" compositions were produced from the average Archaean tholei-ite over the entire experimental range, with 15% to less than 1 % H20. The high-Al basalt produced liquids too high in Al203 (18--23%) for realistic grey gneiss compositions. The persistent generation in our experiments of low-K calc-alkaline magmas directly by vapor-undersaturated partial melting of low-K Archaean tholeiite strongly suggests this mechanism for the origin of early continents. Temperatures of 850°-l000°C and pressures around 15 kbar are appropriate melting conditions. Ton­alitic magmas are favored by higher temperatures, lower pressures, and higher H20 contents in the source. Trondhjemitic magmas are favored by lower temperatures, higher pressures, and lower H20 contents. Heavy REE depletion of magmas would be possible for partial melting above 15 kbar because of the stability at higher pressures of residual garnet. Unfractionated REE patterns of magmas could result from melting at lower pressures, where garnet does not coexist with liquid. The low-K trends of melts are maintained by very refractory amphibole (up to 0.7 wt. % KzO) which coexists with liquid for bulk H20 contents of 2 wt. % or more. Shallow subduction-zone melting of amphibolite with magma extraction, and partial melting of amphibolite under deep-crustal metamorphic conditions are models for early crustal evolution which appear to satisfy the experimental constraints.

Composite Devonian island-arc batholith in the northern Sierra Nevada, California

The Bowman Lake batholith intrudes the pre-Upper Devonian Shoo Fly Complex in the northern Sierra Nevada. The eastern margin of the batholith extends to within 1.5 km of the base of a thick sequence of Paleozoic island-arc rocks that rests unconformably on the Shoo Fly Complex. Hypabyssal silicic intrusions associated with the batholith penetrate the Upper Devonian Sierra Buttes Formation, the lowest volcanic unit of the arc sequence, but do not extend to higher stratigraphic levels. The intrusions in the Sierra Buttes Formation possess marginal peperites and show other evidence of their injection into wet, unconsolidated sediments. On the basis of these field relations, the batholith is interpreted to represent a subvolcanic magma chamber emplaced concurrently with deposition of Sierra Buttes arc rocks. The batholith is composite and consists of trondhjemite, granodiorite, biotite granite, and hornblende tonalite, formed from discrete batches of magma injected into a common plutonic chamber. Mingling of tonalitic and trondhjemitic magmas produced abundant rounded inclusions of tonalite dispersed within trondhjemite and led to hybridization of the two magmas by fine-scale intermixing. Zircons from the trondhjemitic, granodioritic, and granitic phases of the batholith and an associated Sierra Buttes rhyolite sill show complex U-Pb isotopic systematics. A multistage history is suggested, involving the incorporation of earliest Paleozoic and/or Proterozoic zircons during magma generation or ascent in Devonian time and one or more stages of disturbance in Mesozoic-Cenozoic(?) time. Consideration of these different discordance mechanisms results in a model U-Pb igneous age for the batholith of 364 to 385 Ma (Middle to Late Devonian); on the basis of the field relations, a Late Devonian age for the batholith is adopted. Zircon characteristics and isotopic systematics suggest that petrogenesis of the granite involved mixing of trondhjemitic and/or granodioritic magmas with partial melt from a separate, large-ion-lithophile-enriched source.

Combined oxygen isotope – compositional studies of some granitoids from the Grenville Province of Ontario, Canada: implications for source regions

Oxygen isotopic compositions of whole rocks and coexisting quartz–feldspar pairs have been determined for nine pre-, and syn- to late-kinematic granitoid plutons in the Grenville Province of Ontario. These new data demonstrate that granitoid rocks (Algonquin, Mulock) in migmatite terrain of the Ontario Gneiss Segment possess normal δ18O values (<9.0‰), whereas mesozonal to epizonal plutons (Elphin, Coe Hill, Deloro, Barber's Lake) in the Central Metasedimentary Belt (CMB) are characterized by significantly higher 18O contents (δ18O > 9.0‰), in accord with previous results.In the Algonquin sodic suite, a gross covariance of δ18O with compositional indices is present, from 6.4‰, SiO2 = 50.5 wt. % (gabbro) to 8.7‰, SiO2 = 72 wt. % (trondhjemite), resulting from combined assimilation–fractional crystallization. Mafic members of the sodic suite are 18O enriched overall (5.8–7.9‰) relative to fresh tholeiites (5.7 + 0.3‰), implicating some 18O contamination of the protolith. The dispersion of δ18O values in the Algonquin potassic suite, from 4.3 to 9.3‰, is independent of composition and attributed to isotopic exchange with low-18O thermal waters during emplacement. Biotite–hornblende granite of the Mulock batholith is characterized by a limited oxygen isotope compositional range, where the average δ18O = 8.1 ± 0.5‰; δ18O correlates with SiO2 but not with the zonal distribution of Ba, Rb, and Sr abundances.The Union Lake quartz diorite (δ18O = 8.5 ± 0.1‰) and White Lake trondhjemite (δ18O = 7.3 ± 0.6‰) have oxygen isotope compositions comparable to those of other trondhjemitic suites in the CMB. A systematic enrichment of ~1.2‰ in the Union Lake pluton, together with enhanced Ca, Mg, Fe, and Sr, can be accounted for by assimilation of ~5% marbles and 10% amphibolites from the country rock. Uniformly high δ18O values of 11.5 ± 0.8‰ characterize the Elphin granite–syenite complex. The largest values (11.7–12.7‰) and lowest SiO2 (54–56 wt. %) are in the partially assimilated host gabbro–diorite complex, endorsing the presence of 18O-enriched source regions. The Cheddar biotite–hornblende granite, one of a population of intrusions within the alkalic belt of the western CMB, has a restricted isotopic span, where δ18O = 8.8 ± 0.9‰. An unusual concave rare-earth-element (REE) distribution may result from interaction with a heavy rare-earth -element (HREE) enriched volatile phase. The Coe Hill biotite granite (δ18O = 10.4 ± 0.4‰) is isotopically in compliance with other granites and syenites of the CMB. Covariance of δ18O and SiO2, in conjunction with smooth and continuous geochemical trends, is interpreted in terms of assimilation–fractional crystallization.Peralkaline granite of the Deloro pluton includes a hypersolvus phase with high, scattered δ18O values (9.1–11.8‰) and a subsolvus counterpart attributed to late influx of water that induced isotopic reequilibration toward a more constrained range (δ18O = 9.2–10.2‰). REE distributions of a calcic syenite phase are compatible with its evolution by fractional crystallization of a low-K tholeiitic magma, and the high-18O character (δ18O = 11.1–12.6‰) requires 18O enrichment of the protolith and (or) 18O contamination of the magma. Peralkaline rhyolitic volcanics, compositionally coherent with the Deloro pluton and possibly representing extrusive equivalents, possess significantly higher and more variable δ18O values, from 11.7 to 14.2‰; this is attributed to 18O enrichment during low-temperature exchange with thermal waters, superimposed on a primary high-18O magma. The Barber's Lake two-mica granite contains enhanced abundances of U (15 ppm) and Th (36 ppm) in conjunction with systematically elevated δ18O values (10.4 ± 0.5‰). Geochemical constraints are compatible with its evolution from a trondhjemitic magma, but the isotopically enriched nature requires extensive 18O contamination of the protolith and (or) magma. These nine granites variously retain "memory" of primary and (or) secondary features, including δ18O of the source region, covariance of isotopic and compositional parameters, and sporadically superimposed disturbance by exchange with thermal waters. During metamorphism, quartz and feldspar were systematically reset to high-temperature fractionations, but the extent of open-system exchange with rock reservoirs was limited.Despite some probable disturbance by metamorphism and the limited data available, O–Sr isotope systematics of the Grenville granitoids indicate that (1) high-18O granites from the Frontenac Axis were derived from in situ anatexis of Grenville Supergroup metasediments, (2) synkinematic granites were derived by mixing of a primary magma generated at a lower crustal (granulite facies) or upper mantle level with the fusion products generated by partial melting of the Archean–Early Proterozoic type metasediments, and (3) the tonalite–trondhjemite suite in this part of the Grenville Province was derived from a similar lower crustal or upper mantle primary magma by direct fractional crystallization.

Fluid transport in lineaments

Fluid transport in lineaments

Fluid infiltration into fault zones: Chemical, isotopic, and mechanical effects

Fluid infiltration into fault zones and their deeper-level counterparts, brittle-ductile shear zones, is examined in diverse tectonic environments. In the 2.7 Ga Abitibi greenstone belt, major tectonic discontinuities, with lateral extents of hundreds of kilometres initiated as listric normal faults accommodating rift extension and acted as sites for komatiite extrusion and locally intense metasomatism. During reverse motion on the structures, accommodating shortening of the belt, these transcrustal faults were utilised as a conduit for the ascent of trondhjemitic magmas from the base of the crust and of alkaline magmas from the asthenosphere and for the discharge of thousands of cubic kilometres of hydrothermal fluids. Such fluids were characterised by δ18O=+6±2, δD=−50±20, δ13C=−4±4, and temperatures of 270 to 450°C, probably derived from devolatilisation of crustal rocks undergoing prograde metamorphism. Hydrothermal fluids were more radiogenic (87Sr/86Sr=0.7010 to 0.7040) and possessed higher μ than did contemporaneous mantle, komatiites or tholeiites, and thus carried a contribution from older sialic basement. A provinciality of87Sr/86Sr and δ13C is evident, signifying that fault plumbing sampled lower crust which was heterogeneous at the scale of tens of kilometres. Mineralised faults possess enrichments of large ion lithophile (LIL), LIL elements, including K, Rb, Ba, Cs, B, and CO2, and rare elements, such as Au, Ag, As, Sb, Se, Te, Bi, and W. Fluids were characterised by XCO 2≈0.1, neutral to slightly acidic pH, low salinity ≤3 wt-%, K/Na=0.1, they carried minor CH4, CO, and N2, and they underwent transient effervescence of CO2 during decompression. Clastic sediments occupy graben developed at fault flexures. The40Ar/39Ar release spectra indicate that fault rocks experienced episodic disturbance on time scales of hundreds of millions of years. At the Grenville front, translation was accommodated along two mylonite zones and an intervening boundary fault. The high-temperature (580°C) and low-temperature (430 to 490°C) mylonite zones, formed in the presence of deep-level crust-equilibrated fluids of metamorphic origin. Late brittle faults contain quartz veins precipitated from fluids with extemely negative δ18O (−14 per mil) at 200 to 300°C. The water may have been derived from downward penetration into fault zones of precipitation of low18O on a mountain range induced by continental collision, with uplift accommodated at deep levels by the mylonite zones coupled with rebound on the boundary faults. Archean gneisses overlie Proterozoic sediments along thrust surfaces at Lagoa Real, Brazil; the gneisses are transected by brittle-ductile shear zones locally occupied by uranium deposits. Following deformation at 500 to 540°C, in the presence of metamorphic fluids and under conditions of low water-to-rock ratio, shear zones underwent local intense oxidation and desilication. All minerals undergo a shift of −10 per mil, indicating discharge of meteoric-water-recharged formation brines in the underlying Proterozoic sediments up through the Archean gneisses, during overthrusting; ≈1000 km3 of solutions passed through these structures. The shear zones and Proterozoic sediments are less radiogenic (87Sr/86Sr=0.720) than contemporaneous Archean gneisses (0.900), corroborating the transport of fluids and solutes through the structure from a large external reservoir. Major crustal detachment faults of Tertiary age in the Picacho Cordilleran metamorphic core complex of Arizona show an upward transition from undeformed granitic basement through mylonitic to brecciated and hydrothermally altered counterparts. The highest tectonic levels are allochthonous, oxidatively altered Miocene volcanics. This transition is accompanied by an increase of 12 per mil in δ18O, from +7 to +19, and a 400°C decrease in temperature. Lower tectonic levels acted as aquifers for the expulsion of large volumes of higher-temperature reduced metamorphic fluids and/or evolved formation brines. The Miocene allochthon was influenced by a lower-temperature reservoir inducing oxidative potassic alteration; mixing occurred between cool downward-penetrating thermal waters and the hot, deeper aqueous reservoir. In general, flow regimes in these fault and shear zones follow a sequence, from conditions of high temperature and pressure with locally derived fluids at low water-to-rock ratios, during initiation of the structures, to high fluxes of reduced formation or metamorphic fluids along conduits as the structures propagate and intersect hydrothermal reservoirs. Later in the tectonic evolution and at shallower crustal levels there was incursion of oxidising fluids from near-surface reservoirs into the faults. In general, magmatism, tectonics, and fluid motion are intimately related.

Geology, geochemistry and isotopic characteristics of the Archaean Kaap Valley pluton, Barberton Mountain Land, South Africa

Formed approximately 3500 Ma ago, the Kaap Valley tonalite pluton is one of the oldest and the largest of a suite of tonalite-trondhjemite gneiss plutons in the immediate environs of the Barberton greenstone belt. Although similar in many respects to the other members of this suite, the Kaap Valley pluton is distinctively more mafic in composition, containing hornblende instead of biotite as its dominant mafic mineral, and is enriched in rare earth elements. These characteristics are not compatible with an origin simply involving a greater degree of partial melting than that required to form more felsic trondhjemites from the same precursor. The parental magma to the Kaap Valley tonalite is considered, rather, to originally have been a trondhjemitic magma enriched in minor phases such as zircon and apatite, and possibly also dissolved CO2 vapour, which has been contaminated by a mafic component. The form and composition of hornblendes in the tonalite are inconsistent with a xenocrystic origin by incomplete digestion of surrounding mafic metavolcanic country rocks and, consequently, the contamination is inferred to have taken place prior to magma emplacement. Possible mechanisms include magma mixing and complete assimilation at depth of mafic rocks. A subdued form of crystal fractionation occurred during solidification of this magma, resulting in the segregation of discrete hornblende and hornblende + biotite tonalite phases. The tonalite was subsequently diapirically emplaced into its present position, perhaps about 3200 Ma ago, a process which resulted in deformation of the surrounding greenstone belt successions. Diapirism also perturbed the original, probably sub-horizontal, compositional zoning, the present geometry of which can best be explained in terms of an orthogonal pattern of intersecting structural lineaments.

Trondhjemitic basement enclave near the Archean Favourable Lake volcanic complex, northwestern Ontario, Canada

A 15 km2 foliated to gneissic trondhjemite enclave (2.91 Ga) in the Archean North Trout Lake batholith is a metamorphosed remnant of prevolcanism sialic basement. The basement is separated from the nearby Archean Favourable Lake volcanic complex by younger plutons, but was a source of detritus for some of the sedimentary formations in the volcanic complex. The trondhjemite is only a small remnant of an originally more extensive basement.Primary plutonic features can be readily recognized in the trondhjemite in spite of amphibolite facies metamorphism. The basement underwent at least two deformational events: (1) syn-emplacement deformation that produced foliation and gneissosity, and brecciated and deformed early dikes and xenoliths and (2) later recrystallization. The recrystallization was caused by strain and heat produced by emplacement of the North Trout Lake batholith. Emplacement of successive batholith phases progressively heated the basement and elevated it from its originally deeper level. Metamorphism culminated with emplacement of the youngest major phase of the batholith. The intensity of recrystallization and the habit and abundance of late leucocratic quartz monzonite sills and dikes are zoned with respect to this youngest phase.The high Al2O3 trondhjemitic magma probably formed by partial melting of amphibolite at relatively shallow depths. The parent amphibolite probably represents a still earlier volcanic event.

Early Precambrian tonalite-trondhjemite sialic nuclei

Early Precambrian batholiths evolved by diapiric intrusion of near-liquidus to superheated tonalitic and trondhjemitic magmas into an early greenstones crust. Distribution patterns of enclaves and xenolith screens derived from the latter provide reference markers which define the internal geometry and detailed structure of the “gregarious batholiths” (Macgregor) as polydomal multi-lobal bodies. Near-liquidus temperatures are suggested by the digestion of vast volumes of ultramafic—mafic crust by the acid magmas. Tracing of xenolith trains between low and high grade metamorphic terrains provide key evidence for coeval relations between granite—greenstone type terrains and amphibolite to granulite facies infracrustal root zones of the latter. The formation of the plutonic tonalite—trondhjemite suite was accompanied by dacitic to rhyolitic extrusions, the acid volcanic lenses being located above early greenstone units intruded by the batholiths and below upper greenstone sequences which postdate these intrusions. The geochemical characteristics of high-level and deep-level tonalites and trondhjemites are compared. Both suites display very wide compositional spectra, but data from high-grade terrains tend to define a more basic field than data from granite—greenstone terrains. Effects of source compositions on the geochemistry of the acid plutonic rocks are pointed out. Tonalites dominate in South African terrains whereas trondhjemites dominate in Western Australian terrains — a difference conceivably related to the more ultramafic composition of source rocks represented by early greenstone units in southern Africa. Granodiorites and potassic granites form a comparatively minor component of Archaean batholiths, and may occur in the following forms: (1) bands of augen gneiss in high-grade terrains; (2) components of trondhjemitic to granitic gneisses in high-level plutons; and (3) discrete post-tectonic intrusions typically emplaced at high levels of the batholiths and along older tonalite—greenstone contacts. Migmatites characteristically form in close spatial association with xenolith-rich zones, probably due to depression of the solidus consequent on water addition related to dehydration of the xenoliths. A derivation of the acid sodic magmas by anatexis of sialic materials is inconsistent with geochemical evidence and petrological theory. In contrast, the commonly low to very low LIL element levels and REE evidence indicate derivation by about 30–50% melting of basic rocks. Marked trace element anomalies are characteristic of some Archaean plutonic suites, e.g. very high Sr in some Western Australian rocks, low Rb in some Lewisian (Scotland) and South African rocks, U depletion in South African and southwestern Greenland suites, high Li in some Pilbara rocks and high Zr in some southwestern Greenland rocks. However, the only consistent anomaly observed to date is a well-pronounced depletion in Y and heavy REE, suggesting extensive equilibration of the acid melts with eclogite and/or amphibolite. Uniformitarian interpretations of the Archaean are questioned in the light of the evidence for high temperature and pressure, the unique tectonic style of diapirism and the low initial 87Sr/ 86Sr as compared to Proterozoic plutonic suites. The diachronous nucleation of tonalite—trondhjemite plutons during the Archaean is seen as the major process effecting a transformation of an early Archaean sima into sial.

Geochemistry of the Gabbro--Diorite--Tonalite--Trondhjemite Suite of southwest Finland and its Implications for the Origin of Tonalitic and Trondhjemitic Magmas

Geochemistry of the Gabbro--Diorite--Tonalite--Trondhjemite Suite of southwest Finland and its Implications for the Origin of Tonalitic and Trondhjemitic Magmas

Magma mixing in the Flavrian pluton, Noranda area, Quebec

The Archean Flavrian pluton is composed of quartz gabbroic, trondhjemitic, tonalitic and hybrid rocks. Field relationsips indicate that, when trondhjemitic magma was emplaced, some quartz gabbro was solid and some was still mobile.The tonalites are homogenous rocks which were formed when quartz gabbroic and trondhjemitic magmas mixed, largely by a process of diffusion down chemical potential gradients. The most characteristic feature of the tonalites is the presence of large, elongated, branching and curved pyroxene crystals. These crystals owe their unusual size and habits to high levels of pre-nucleation supersaturation, a condition which in turn resulted from the mixing of magmas.The hybrid rocks are heterogeneous mixtures of quartz gabbro, tonalite and trondhjemite. They were produced when quartz gabbroic and trondhjemitic magmas mixed turbulently.Because they are thought to indicate the paleotectonic environment, field relationships indicative of coexisting acid and basic magmas should be routinely recorded during mapping.

The 1.7- to 1.8-b.y.-old trondhjemites of southwestern Colorado and northern New Mexico: Geochemistry and depths of genesis

Four trondhjemitic bodies — three of intrusive and one of extrusive origin — 1.7 to 1.8 b.y. in age occur in the Precambrian rocks of northern New Mexico and southwestern Colorado. These are the metamorphosed plutonic or hypabyssal trondhjemite of Rio Brazos, New Mexico, the interlayered quartzofeldspathic and metabasaltic metavolcanic Twilight Gneiss of the West Needle Mountains, Colorado, the syntectonic Pitts Meadow Granodiorite of the Black Canyon of the Gunnison River, Colorado, and the late syntectonic to posttectonic Kroenke Granodiorite of the Central Sawatch Range, Colorado. From south to north, over a distance of 235 km, the four rock units show systematic increases in average Al2O3 from 13.7 to 16.1 percent, in K2O from 1.5 to 2.6 percent, in Rb from 28 to 76 ppm, and in Sr from 101 to 547 ppm. Initial Sr87/Sr86 ratios are low — 0.7015 to 0.7027 — and suggest a mafic or ultramafic source. All four trondhjemite bodies have similar light rare-earth element (REE) contents. The trondhjemites of Rio Brazos and the Twilight Gneiss have relatively flat patterns (Ce/Yb 10) with low heavy rare earth content and small or no Eu anomalies. Whole-rock δO18 values for siliceous rocks of three of the bodies range from 5.8 to 8.0 per mil, although the Pitts Meadow Granodiorite gives values of 8.5 to 9.4 per mil. The parent magmas for these bodies were probably generated from a parental basaltic source, either by partial melting or fractional crystallization. Fractional crystallization mechanisms would operate at crustal levels where crystallization of plagioclase and clinopyroxene or hornblende would produce the Rio Brazos and Twilight magmas, and crystallization of hornblende, plagioclase, and biotite would produce the Kroenke and Pitts Meadows magmas. The preferred partial melting mechanism would produce the Rio Brazos and Twilight magmas at shallow depth (< 50 km), leaving a residue of plagioclase and clinopyroxene or amphibole; the Pitts Meadow magma at 50 to 60 km, where hornblende, garnet, clinopyroxene, and plagioclase would be residual; and the Kroenke magma at greater than 60 km leaving a residue of garnet and clinopyroxene. The magmas probably formed in a ridge-and-basin complex that lay between the early Precambrian craton to the north and the contemporaneous quartzite-rhyolite-tholeiite terrane to the south. A northward-dipping subduction zone can be postulated from the variation in compositions and inferred depths of melting, but complete modern analogues of similar setting are not known. A better tectonic analogue might be the Archean regimes, in which vertical motion is dominant and trondhjemitic magmas may have formed by melting at the base of foundering thick volcanic piles.

Geochemische Untersuchungen zur Petrographie des Cima d'Asta-Plutons

The chemical composition of the various rocks of the Cima d'Asta Intrusion were investigated. On the basis of 29 analyses it is concluded that the granodioritic rocks of the central area of the intrusion belong to the trondhjemitic, leuco-granodioritic and granodioritic magma types, whereas the quartzdioritic rocks of the southwestern border belong to melaquartzdioritic to quartzdioritic magma types. Xenoliths occuring in the top region of the Cima d'Asta massive are chemically and mineralogically identical with the quartzdioritic rocks of the southwestern border.

Petrologic Study of Bald Rock Batholith, near Bidwell Bar, California

Compton made a detailed study of the Bald Rock batholith, California, and suggested that the tonalites and granodiorites of the rim of the batholith originated through contamination. He believed that trondhjemite magma, represented by rocks in the core of the batholith, assimilated meta-basaltic country rocks. This paper reports a detailed study of 22 rocks from the batholith, including all the main rock types. Values for Qu, Or, Ab, and An calculated from the modes show that the rocks fall along a trend slightly below the cotectic boundary between quartz and feldspars and directed toward this surface. Compositions calculated from the modes indicate that formation of the tonalites by contamination is possible but requires improbably high proportions of metabasalt xenoliths in the rim of the pluton. Habits of zircons separated from the rocks show a distribution pattern which emphasizes contrasts between trondhjemitic core and granodiorite-tonalite rim of the batholith but does not support the idea of formation of the mantle rocks by contamination. Dimensional zircon data indicate that zircons of the core are unimodal, whereas those of the mantle of the batholith are bimodal. Comparisons with the Bald Mountain batholith, Oregon, demonstrate many similarities, as well as some striking contrasts. Compositional trends of tonalite to granodiorite are different for the two plutons. Zircons are the same throughout the Bald Mountain batholith. The writers conclude that both plutons were emplaced by forceful injection. At Bald Mountain, Oregon, a uniform magma was injected, but at Bald Rock, California, a migma-magma was emplaced. The trondhjemitie core of the Bald Rock batholith was mainly liquid at emplacement, but the tonalite-granodiorite rim consisted of mobile, viscous solid phases with interstitial melt. The liquid line of descent at Bald Mountain, Oregon, was from tonalite to granodiorite and at Bald Rock, California, only from trondhjemite to leucotrondhjemite.

TRONDHJEMITE BATHOLITH NEAR BIDWELL BAR, CALIFORNIA

Bald Rock batholith, California, is one of a chain of small plutons that lies 20 miles west of the main Sierra Nevada pluton and parallels the N.15°W. structural trend of the northern Sierra. The country rocks are mainly late Paleozoic (?) basic volcanic rocks cut by small masses of basic plutonic rocks prior to late Jurassic time and folded and dynamothermally metamorphosed in the Nevadan orogeny. Foliation in the pre-batholith terrain was vertical, and its mineral assemblages were, with local anomalies, typical of greenschist facies. After regional metamorphism, gabbroic, dioritic, and granitic rocks were emplaced south and east of the site of Bald Rock batholith. The basic rocks of this sequence are in part magmatic, but the granitic bodies show gradational borders and internal structures that indicate static metasomatic growth, perhaps related to magmatic cores. Bald Rock batholith was a forceful intrusion, as is shown by the abrupt concordant swing of bedding around it. However, local crosscutting relations show that about a fourth of its area at the exposed level was gained by other means. Large concentric outliers and a hull of injection migmatite suggest stoping. Inclusions are too scarce to account for the missing wall rocks, but gradational zoning of the batholith from a trondhjemite core through granodiorite to a heterogeneous tonalite rim suggests that basic stoped rock contaminated an originally trondhjemitic magma. This is supported by comparing a computed chemical composition of the granodiorite and tonalite with chemical analyses of two trondhjemites. The benign nature and pattern of flow structures shows that the different granitic rocks could not have been intruded separately. In contrast to the wall rock, plagioclase of the granitic rocks shows pronounced oscillatory zoning, indicating thorough reworking of the stoped rock. The reactions required for the changes were presumably exothermic, but the broad mixing shown here must have required additional heat. Contact-metamorphic rocks of epidote amphibolite to possible pyroxene hornfels facies form an aureole that had an area nearly as great as the original kinetic intrusion. Squeezing and vertical stretching of these rocks is indicated by pebble and cyrstal lineation, most pronounced near the contact. Post-kinetic replacement veins of tonalite occur sporadically throughout the inner part of the aureole. The rise in anorthite content of plagioclase in metabasaltic rocks is gradual toward the batholith and sharp near the tonalite veins. The obvious fracture control of these high-temperature metamorphic structures indicates that heat addition to the aureole was largely through fluid transfer rather than simple thermal diffusion.