Melting Of Continental Crust Research Articles

Time constraints on the inversion of the tectonic regime in the northern margin of the North China Craton: Evidence from the Daqingshan granites

The Daqingshan granites are located in a late Mesozoic tectono-magmatic belt at the northern margin of the North China Craton (NCC), and include the Deshengying, Xinisubei, Gulouban, and Kuisu plutons. Ion probe U–Pb zircon dating indicates that the granites were emplaced at 131±1, 140±4, 145±1, and 142±2Ma, respectively. All of the granites are alkali- and potassium-rich, with high SiO2 (73.2–76.7wt.%), K2O (4.50–5.57wt.%), Na2O (3.60–4.93wt.%), and K2O/Na2O (0.99–1.49), and low Al2O3 (12.3–14.5wt.%), CaO (0.45–0.79wt.%), and MgO (⩽0.12wt.%). The granites are light rare earth element enriched ([La/Yb]N=5.6–48.7). The Xinisubei and Gulouban monzogranites and the Kuisu mylonitic monzogranite have small Eu anomalies (δEu=0.65–1.23), low Zr+Nb+Ce+Y (132–321ppm), and exhibit a negativecorrelation between P2O5 and SiO2 contents, which are characteristic of highly fractionated I-type granites with a post-collisional origin. The Deshengying monzogranite is distinctive in being an aluminous A-type granite as evidenced by high 10,000×Ga/Al (>2.6) and Zr+Nb+Ce+Y (312–532ppm), low Ba and Sr, marked negative Eu anomalies (δEu=0.08–0.20), strong Ba, Sr, P, and Ti depletions, and an absence of alkali minerals. This granite was probably produced by partial melting of continental crust heated by hot mantle-derived magmas during crustal extension. The Deshengying monzogranite represents a post-kinematic pluton emplaced into the Daqingshan fold-and-thrust belt, whereas the Kuisu mylonitic monzogranite is a syn-kinematic pluton intruded along the Hohhot detachment fault. It is evident that the Daqingshan area experienced a change from a compressional to an extensional tectonic regime during 145–140Ma. The post-orogenic collapse may have resulted in extension of the upper continental crust. Subsequently, as the thrust-detachment system became inactive, the lower crust of the NCC underwent modification and melting from 131Ma. We conclude that the Early Cretaceous tectonic evolution of the Daqingshan area was caused by post-orogenic collapse and melting of the lower crust of the NCC. Delamination of the lower crust in the northern NCC resulted in crustal extension and asthenospheric upwelling, which produced A-type granites. As such, melting of the lower crust in the northwestern part of the NCC took place as early as the late Mesozoic.

South China continental margin signature for sandstones and granites from Palawan, Philippines

We report results of heavy mineral analysis and U–Pb dating of detrital zircons from metasediments and Cenozoic sandstones, and U–Pb dating of zircons from Cenozoic granites of the North Palawan Continental Terrane (NPCT) and the South Palawan Terrane (SPT). The NPCT metasediments are derived mainly from granitic and metamorphic rocks of continental character. They contain zircons that indicate a maximum depositional age of Late Cretaceous and other age populations indicating a South China origin. The sediments were deposited on the South China margin before rifting of the continental margin during opening of the South China Sea. Miocene SPT sandstones contain similar heavy mineral assemblages suggesting sources that included NPCT metasediments, metamorphic basement rocks at the contact between the SPT and the NPCT, South China Sea rift volcanic and/or minor intrusive rocks, and the Palawan ophiolite complex. The SPT sandstones are very similar to Lower Miocene Kudat Formation sandstones of northern Borneo suggesting a short-lived episode of sediment transport from Palawan to Borneo in the Early Miocene following arc-continent collision. U–Pb dating of zircons shows that the Central Palawan granite is Eocene (42±0.5Ma). The Capoas granite was intruded during a single pulse, or as two separate pulses, between 13.8±0.2Ma and 13.5±0.2Ma. Inherited zircon ages from the Capoas granite imply melting of continental crust derived from the South China margin with a contribution from Cenozoic rift-related and arc material.

Mineralogical and geochemical evidence of magma mingling/mixing in the Sierra de las Cruces volcanic range, Mexican Volcanic Belt

Pliocene – Pleistocene lava flows, mainly of dacitic composition, are exposed in the Sierra de las Cruces (SC) volcanic range within the Mexican Volcanic Belt (MVB). SC volcanic rocks are porphyritic, generally containing an assemblage of plagioclase + amphibole + orthopyroxene ± clinopyroxene ± quartz ± Fe-Ti oxides. Most of them exhibit diverse mineralogical and geochemical features that attest a magma mixing and mingling processes with concomitant fractional crystallization in which a small volume of hot andesite magma injects into dacitic magma. Both rock types are probably derived from partial melting of continental crust at different levels. The evidences of magma mixing and mingling include: (a) normal and sieved plagioclases in the same sample, rounded and embayed crystals, and armoured rims over the dissolved crystal surfaces; (b) subrounded, vesicular magmatic enclaves, ranging from a few millimeters to ~20 centimeters in size, with plagioclase + orthopyroxene + amphibole + quartz ± olivine ± Fe-Ti-oxides assemblage; (c) mineral chemistry evidence such as crystals with reaction rims or heterogeneous plagioclase compositions (inverse and oscillatory zoning or normally and inversely zoned crystals) in the same sample; and (d) elemental geochemical variations and trace-element ratio more akin to magma mixing and to some extent diffusion process. These andesitic enclaves could be considered as portions of the intermediate magma that did not mix completely (mingling) with the felsic host lavas, confirming the major role of magma mixing and mingling processes in the overall evolution of the MVB

Petrogenesis and Solidification Depth of the Jurassic Daebo and Cretaceous Bulguksa Granitic Rocks in South Korea

AbstractWe investigated the Jurassic Daebo and Cretaceous Bulguksa granitic rocks in South Korea. The former are distributed mainly in the Gyeonggi and Yeongnam massifs and the latter are present in the Gyeongsan basin and Ogcheon belt. The Daebo granitic rocks generally are of ilmenite series and I to S type. These rocks are associated with Au–Ag hydrothermal deposits, whereas the Bulguksa granitic rocks are of magnetite series and I type, and are associated with Pb–Zn, Cu and Mo–W hydrothermal deposits, as well as Au–Ag hydrothermal deposits. The Daebo granitic rocks show adakitic signatures in their chemical compositions. They are considered to have been derived from partial melting of the thick lower continental crust. Conversely, the Bulguksa granitic rocks in the Gyeongsan basin are non‐adakitic and are considered to have been derived from partial melting of a mantle wedge. Magmas of the Daebo granitic rocks formed at relatively shallow levels, but solidified at deep levels compared with those of the Bulguksa granitic rocks. The Bulguksa granitic rocks in the central to western Ogcheon belt are considered to have been formed by fractionation of magmas derived from partial melting of continental crust. The total Al contents of biotite and hornblende in the granitic rocks increased, with the Bulguksa granitic rocks in the Gyeongsan basin < the Bulguksa granitic rocks in the Ogcheon belt and Gyeonggi and Yeongnam massifs and the Daebo granitic rocks in the Ogcheon belt < the Daebo granitic rocks in the Gyeonggi and Yeongnam massifs. This order corresponds to an increase in solidification depth.

Tuff beds in Kurnool subbasin, southern India and implications for felsic volcanism in Proterozoic intracratonic basins

A first report on tuff beds from the Owk Shale in the Proterozoic Kurnool sub-basin in southern India is presented. The rhyolitic to rhodacitic tuffs, overlying shelfal limestones formed at depths below storm wave base, have rheomorphic features indicative of viscoplastic flow, and geochemical signatures of rhyolitic to rhyodacitic unwelded to welded tuffs, similar to those described from other Proterozoic intracratonic basins like Vindhyan and Chhattisgarh basins in India. Fragmentary nature of altered glass with perlitic cracks and local admixture with intrabasinal sediments suggest phreatomagmatic reactions. The widespread and repeated occurrences of felsic tuffs in these basins, possibly derived from low degree melting of continental crust, suggest intermittent tectonothermal instability which likely influenced basinal topography and cyclic development of the carbonate platforms.

Geochemical Constraints on Adakites of Different Origins and Copper Mineralization

AbstractThe petrogenesis of adakites holds important clues to the formation of the continental crust and copper ± gold porphyry mineralization. However, it remains highly debated as to whether adakites form by slab melting, by partial melting of the lower continental crust, or by fractional crystallization of normal arc magmas. Here, we show that to form adakitic signature, partial melting of a subducting oceanic slab would require high pressure at depths of >50 km, whereas partial melting of the lower continental crust would require the presence of plagioclase and thus shallower depths and additional water. These two types of adakites can be discriminated using geochemical indexes. Compiled data show that adakites from circum-Pacific regions, which have close affinity to subduction of young hot oceanic plate, can be clearly discriminated from adakites from the Dabie Mountains and the Tibetan Plateau, which have been attributed to partial melting of continental crust, in Sr/Y-versus-La/Yb diagram. Given t...

Collisional granitoids of the Dzhida zone of the Central Asian Fold belt, Southwestern Transbaikalia: Age and conditions of the formation

This paper considers the geological structure, composition, and age of the Darkhintui, Barun-Gol, and Khuldat granitoid plutons of the Dzhida zone of Caledonides of the Central Asian Fold belts. These plutons were formed in the Late Cambrian-Early Ordovician in the range between 490 ± 2 and 477 ± 6 Ma, after tectonic juxtaposition of the oceanic and island-arc complexes of the Dzhida Zone and volcanogenic-carbonate-terrigenous rocks of the Khamardaban zone, i.e., at the collisional stage of the region evolution. Geological, geochronological, geochemical, and Nd isotope data indicate that the collisional granitoids of the Dzhida zone were derived by melting of continental crust thickened through accretion. The sources for parental melts of the granitoids were presumably Vendian-Early Cambrian juvenile igneous rocks of ophiolite and island-arc complexes, as well as the crustal material of the Lower Paleozoic flyschoid sediments of the back-arc basin of the Dzhida zone and metaterrigenous rocks of the Khamardaban zone.

Zircon ages and geochemistry of late Neoarchean syenogranites in the North China Craton: A review

At the end of the Neoarchean continental evolution, voluminous syenogranites were emplaced in the North China Craton, together with other magmatic rocks (trondhjemite–tonalite–granodiorite (TTG), monzogranite, diorite, gabbro). Syenogranites are widely distributed in Anshan-Benxi, Qinhuangdao and western Shandong, and also occur in southern Jilin, northern Liaoning, northwestern Hebei and central Henan. Based on geological relationships, degree of metamorphism, deformation and magmatic zircon ages, two phases of syenogranite magmatism are recognized. Rocks produced during the first phase show a gneissic texture and were formed between 2.53 and 2.52Ga and locally comprise abundant TTG. Rocks of the second phase cut late Neoarchean TTG and supracrustal rocks, display a massive structure, and mainly formed between 2.52 and 2.50Ga. All syenogranites share the same features in major element compositions, being high in SiO2 and low in CaO, total FeO, MgO, TiO2 and P2O5. However, they are different in trace and REE compositions and can be subdivided into three types. (1) Type 1 shows a large variation in total REE contents, low (La/Yb)n ratios, strong negative Eu*/Eu anomalies and Ba depletion; (2) Type 2 is similar to Type 1 but has higher (La/Yb)n ratios. (3) Type 3 shows a large variation in total REE and (La/Yb)n ratios and significantly do not show strongly negative Eu*/Eu anomalies and Ba depletion. Whole-rock Sm–Nd isotopic compositions show large variations in ɛNd(t) values and tDM(Nd) modal ages, ranging from −9.49 to −4.72 and 3.70 to 3.25Ga (Type 1), 0.55–1.03 and 2.77–2.71Ga (Type 2) and −2.35 to 1.23 and 2.93–2.66Ga (Type 3), respectively. Hf isotopic compositions of zircons from three samples have ɛHf(t) values and tDM1(Hf) ages of 0.7–7.2 and 2.84–2.56Ga (Type 1), 2.6–7.4 and 2.74–2.56Ga (Type 2) and 2.1–6.3 and 2.76–2.60Ga (Type 3). It is concluded that syenogranites were generated by melting of continental crust with different mean crustal residence ages, and most of them were emplaced during the second phase (2.52–2.50Ga) in an extensional tectonic regime. The formation of these voluminous syenogranites marks a tectono-magmatic event resulting in stabilization of the North China Craton at the end of the Neoarchean.

Composition of the Marginal Rocks and Sills of the Rustenburg Layered Suite, Bushveld Complex, South Africa: Implications for the Formation of the Platinum-Group Element Deposits

The Bushveld Complex contains large ore deposits of platinum-group elements (PGE), V, and Cr. Understanding how these deposits formed is in part dependent on estimates of the compositions of the magmas that filled the Bushveld chamber. Over the past 20 years, estimates for the major oxides and some trace elements in the magmas have been made using the marginal rocks of the intrusion. However, data for most of the trace elements have not been available. This paper presents the results for a full range of trace elements, including the platinum-group elements. The marginal rocks of the Lower and lower Critical zones (B-1 magmas) are tholeiitic Mg-rich basaltic andesites with Mg# 71. It had been suggested that they are boninites but their mantle-normalized incompatible lithophile trace element patterns (spidergrams) resemble those of the upper continental crust and the concentrations of the elements are much higher than those of boninites. The patterns resemble siliceous high magnesium basalts. An unusual feature is that the Pt/Pd ratios are >1.5. The Pt contents of the B-1 rocks (15–25 ppb) are slightly higher than those observed in most primary mantle melts, suggesting that the high Pt/Pd ratio is due to Pt enrichment rather than Pd depletion. The crystallization order and composition of the minerals formed in equilibrium with the B-1 magma matches that of the Lower and lower Critical zones and thus this magma appears to be representative of the parental magma of these zones. The marginal rocks to the upper Critical zone (B-2) are tholeiitic basalts in terms of major element composition, with Mg# 55. The spidergrams show some similarities with E-MORB; however, the B-2 rocks have strong positive Ba and Pb anomalies and negative P, Ti, Hf, and Zr anomalies, and thus they more closely resemble lower continental crust. The B-2 rocks have lower and more variable Pt + Pd contents than the B-1 magma, suggesting that some of the samples have experienced sulfide saturation, but in common with the B-1 magmas, the Pt/Pd ratios are high, in excess of 1.5. The crystallization order of the Upper Critical zone cannot be modelled by the B-2 magma alone. However, mixtures of B-2 magma and B-1 magma satisfy the crystallization order and mineral composition of the upper Critical zone. The marginal rocks of the Main zone (B-3) are also tholeiitic basalts in terms of major element composition but have a higher Mg# (62) than the B-2 rocks. Trace element patterns in part resemble those of B-2 magmas but are depleted in most incompatible elements with large positive Ba, Pb, and Eu anomalies and negative Nb, Ta, Hf, and Zr anomalies, suggesting the rocks contain a plagioclase component. The PGE contents of the B-3 rocks are lower than those of the B-1 magma and less variable than those of the B-2 magma, but in common with both the other magmas, they have high Pt/Pd. The crystallization order and composition of the minerals in equilibrium with the B-3 magma matches that of the Main zone. Two processes have been suggested to explain the compositions of the Bushveld magmas: mixing of primitive mantle melts with partial melts of continental crust and mixing of primitive mantle melts with melts derived from the subcontinental lithospheric mantle (SCLM). The trace element concentrations of the magmas can be modelled by crustal contamination. This interpretation is supported by oxygen isotopes, initial 87Sr/86Sr ratios and ɛNd of the cumulate rocks. However, the high Pt/Pd ratios of all of the magmas and the overall higher than normal Pt concentrations of the B-1 magma are difficult to explain by mixing of primary mantle melt with crustal components. The SCLM has high Pt/Pd ratios and mixing of primary mantle magma with SCLM-derived magma could account for the high Pt concentrations and high Pt/Pd ratios. This interpretation is supported by recent work on Os isotopes of the Kaapvaal SCLM. It should be kept in mind that the two processes are not necessarily exclusive. A magma with a SCLM component could have been emplaced into the crust and subsequently have been contaminated by partial melts of the crust.

Transport properties of low-sanidine single-crystals, glasses and melts at high temperature

Thermal diffusivity (D) was measured using laser-flash analysis from oriented single-crystal low-sani- dine (K0.92Na0.08Al0.99Fe 3+ Si2.95O8), and three glasses near KAlSi3O8. Viscosity measurements of the three super- cooled liquids, in the range 10 6.8 to 10 12.3 Pa s, confirm near- Arrhenian behavior, varying subtly with composition. For crystal and glass, D decreases with T, approaching a constant near 1,000 K: Dsat * 0.65 ± 0.3 mm 2 s -1 for bulk crystal and *0.53 ± 0.03 mm 2 s -1 for the glass. A rapid decrease near 1,400 K is consistent with crossing the glass transition. Melt behavior is approximated by D = 0.475 ± 0.01 mm 2 s -1 . Thermal conductivity (klat) of glass, calculated using previous heat capacity (CP) and new density data, increases with T because CP strongly increases with T. For melt, klat reaches a plateau near 1.45 W m -1 K -1 , and is always below klat of the crystal. Melting of potassium feldspars impedes heat transport, providing positive thermal feedback that may promote further melting in continental crust.

Alteration and remelting of nascent oceanic crust during continental rupture: Evidence from zircon geochemistry of rhyolites and xenoliths from the Salton Trough, California

Rhyolite lavas and xenoliths from the Salton Sea geothermal field (Southern California) provide insights into crustal compositions and processes during continental rupture and incipient formation of oceanic crust. Salton Buttes rhyolite lavas contain xenoliths that include granophyres, fine-grained altered rhyolites (“felsite”), and amphibole-bearing basalts. Zircon is present in lavas and xenoliths, surprisingly even in the basaltic xenoliths, where it occurs in plagioclase-rich regions interpreted as pockets of crystallized partial melt. Zircons in the xenoliths are exclusively Late Pleistocene–Holocene in age and lack evidence for inheritance. U–Th isochron ages are: 20.5 − 1.2 + 1.2 ka (granophyres), 18.3 − 3.5 + 3.6 ka (felsite), 30.1 − 12.4 + 14.1 ka and 9.2 − 6.6 + 7.0 ka (basalts; all errors 1 σ). The dominant zircon population in the rhyolite lavas yielded U–Th ages between ∼ 18 and 10 ka, with few pre-Quaternary xenocrysts present. δ 18O zircon values are lower than typical crustal basement values, thus ruling out rhyolite genesis by melting of continental crust. Moreover, δ 18O zircon values are ∼ 0.5–1.0‰ lower than compositions achievable by zircon crystallization from residual melt in equilibrium with unaltered mid-ocean ridge basalt, suggesting that basaltic crust and silicic plutons in the subsurface of the Salton Sea geothermal field isotopically exchanged with meteoric waters. This is evidence for deep-reaching hydrothermal circulation and indicates rhyolite genesis by episodic remelting of altered basalts instead of fractional crystallization of unaltered basaltic magma.

Geochemistry and U-Pb zircon geochronology of Late-Mesozoic lavas from Xishan, Beijing

Zircon U-Pb dating by both SHRIMP and LA-ICP-MS and geochemical study of the Tiaojishan Formation and the Donglintai Formation from Xishan, Beijing, reveal that ages of upper lavas of Tiaojishan Formation and Middle of Donglintai Formation are 137.1±4.5 Ma(2σ) and 130－134 Ma, respectively. The fomer is slightly older than the latter and the age difference between these two formations is less than 5 Ma. These lines of evidence prove that the two volcanoes erupted within a short time. The age of the Tiaojishan Formation from Xishan, Beijing is distinctively different from that of the Chende Basin. This indicated that the ages of Tiaojishan lavas varied in different regions. The Tiaojishan Formation consists of typical adakite (SiO 2 =56%, Na 2 O = 3.99－6.17, Na 2 O/K 2 O = 2.2－3.1, Sr = 680－1074´10 -6 , Y = 13.2－16.3´10 -6 , Yb = 1.13－1.52´10 -6 , Sr/Y = 43－66), high-Mg adakite and high-Mg andesite (Mg# = 54－55). Features of continental crust of adakite from the Tiaojishan Formation and its syngeneric middle silicic vocanic rocks, such as typical Nd-Ta negative abnormality and Pb possive abnormality, indicate that these lavas are originated from partial melts of continental crust. These results suggest that the adakite from the Tiaojishan Formation of Xishan, Beijing derived from thickened eclogitic lower crust and lithosphere beneath the North China craton at mesozoic that was foundered into the aesthenosphere, and subsequenctly partially melted and interacted with mantle olivine during melts upward migration. The age of lavas from the Tiaojishan Formation restrained the foundation which should last at least until 137 Ma. Lavas of the Donglintai Formation are rhyolith and andesite with normal Mg# and thus they did not interact with the mantle. These lavas represent remobilized melts of lower crust material caused by mantle aesthenosphere upwelling migration induced by foundation.

Granites and granitic magmas: strange phenomena and new perspectives on some old problems

Granitic plutons generally afford pleasant, mostly unchallenging landscapes. However, on closer inspection of some of the outcrops, one may discover some strange, beautiful and baffling examples of patterns produced by geochemical and mineralogical self-organization. Fascinating as they are, these features probably reveal little about the origins of granitic magmas. It is the more usual features that one needs to study and understand, using geophysics, geochemistry, isotope petrology, textural analysis and various kinds of theoretical modelling. Many granites are products of high-temperature melting of continental crust and there is a relationship between upper granulite-facies metamorphism and the generation of granitic magmas. The heat needed for this process commonly comes from mafic magmas intruded into the deep crust and the melting reactions take place in the absence of free fluids. The relatively low viscosity and density of hydrous granitic magmas control the ascent and emplacement processes and the viscosity is not greatly changed during crystallization and cooling, at least for the first few tens of percent crystals. Modelling shows that granite diapirism is slow and inefficient and there is little evidence of it in the geological record. The granitic melt segregates into veins, shears and dykes, eventually forming larger feeder dykes that transport the magma rapidly upward to the emplacement sites. The potential speed of ascent and emplacement mean that even very large granitic plutons probably grow in thousands of years. Experimental data on crystallization rates suggest that many of the large crystals observed in granitic rocks could have grown in a matter of hours – certainly in no more than a few tens of years. Geological processes are commonly thought of as slow and continuous, but many are rapid and episodic. Granitic plutonism is of the latter kind.

Tectonic setting and regional correlation of Ordovician metavolcanic rocks of the Casco Bay Group, Maine: evidence from trace element and isotope geochemistry

Ordovician metamorphic rocks of the Casco Bay Group are exposed in an approximately 170 km long NE-trending belt (Liberty-Orrington belt) in southern and south-central Maine. Geochemical analysis of rocks within the Spring Point Formation (469±3 Ma) of the Casco Bay Group indicate that it is an assemblage of metamorphosed bimodal volcanic rocks. The mafic rocks (originally basalts) have trace element and Nd isotopic characteristics consistent with derivation from a mantle source enriched by a crustal and/or subduction component. The felsic rocks (originally rhyolites and dacites) were likely generated through partial melting of continental crust in response to intrusion of the mafic magma. Relatively low initial εNd values for both the mafic (−1.3 to +0.6) and felsic (−4.1 to −3.8) rocks suggest interactions with Gander zone continental crust and support a correlation between the Casco Bay Group and the Bathurst Supergroup in the Miramichi belt of New Brunswick. This correlation suggests that elements of the Early to Middle Ordovician Tetagouche-Exploits back-arc basin can be traced well into southern Maine. A possible tectonic model for the evolution of the Casco Bay Group involves the initiation of arc volcanism in Early Ordovician time along the Gander continental margin on the eastern side of the Iapetus Ocean basin. Slab rollback and trenchward migration of arc magmatism initiated crustal thinning and rifting of the volcanic arc around 470 Ma and resulted in the eruption of the Spring Point volcanic rocks in a back-arc tectonic setting.

Isothermal decompression, partial melting and exhumation of deep continental crust

Abstract Decompression of deep, hot continental crust is the primary mechanism of crustal melting, with major consequences for the geodynamics of orogens. Decompression within thickened continental crust may be initiated by processes driven from above (erosion, tectonic denudation) and/or below (crust/lithosphere thinning, buoyant rise of deep crust). On a larger scale, decompression of subducted continental crust may add material, including melt, to the overlying, non-subducting plate. This mechanism has the potential to produce large amounts of melt because fertile material is continually conveyed into the mantle, where it eventually buoyantly ascends and melts. Decompression-driven melting of continental crust may account for the high melt fractions (≥20 vol.%) and great thickness (20–30 km) inferred for the partially molten layer in orogenic crust. When high melt volumes are present in the crust and/or the thickness of the partially molten layer is large, the subsequent thermo-mechanical evolution of orogens is strongly influenced by lateral (channel) and vertical (buoyant) crustal flow. For both lateral and vertical flow, the presence of melt decouples deep crust from upper crust, and continental crust from mantle lithosphere. A major consequence of vertical crustal flow is the generation of migmatite-cored gneiss domes that riddle most orogens. High-grade rocks in many domes record pressure-temperature-time ( P-T-t ) paths indicating near-isothermal decompression followed by cooling from T > 700 °C to T < 350 °C in <2–5 Ma. Diapiric ascent of partially molten crust accounts for the decompression rate and magnitude required to maintain a near-isothermal path. We propose that gneiss domes are a signature of decompression and crustal melting, and are therefore fundamental structures for understanding the thermo-mechanical evolution of continental crust during orogeny.

Late Archaean granites: a typology based on the Dharwar Craton (India)

Abstract Extensive field work in the Eastern Dharwar Craton, associated with petrographic and geochemical (major and trace elements) investigations, allows four main types of Late Archaean granitoids to be distinguished. (1) Na-rich granitoids of trondhjemitic, tonalitic and granodioritic composition (TTG) that are characterised by strongly fractionated REE patterns and low HREE contents and generally interpreted as “slab melts” generated by partial melting of metamorphosed hydrated basalt, most likely in a subduction environment. (2) Sanukitoids, which are K- and Mg-rich monzonites and granodiorites with TTG-like REE patterns associated with marked LILE-enrichment, and considered to result from the reaction of slab melts generated in a subduction environment with, and assimilation of, mantle wedge peridotite. (3) Uncommon high-HFSE, Mg and K granites with strongly REE and LILE-enrichment that, probably formed by partial melting of an enriched mantle source; unlike in the genesis of sanukitoids, in this case the slab melt is considered to be wholly consumed by reaction with mantle minerals, resulting in mantle-enrichment. Subsequent melting of this enriched mantle (probably in a post-subduction setting) gives rise to high-HFSE, Mg and K magmas. As demonstrated for the Closepet Granite (Dharwar Craton), the hot mantle-derived magma can induce melting of continental crust and then mix with the anatectic products. (4) K-rich, Mg-poor anatectic biotite-granites with REE patterns that are less fractionated and show negative Eu anomalies. These granites result from the remelting of old basement or recently accreted plutons, both with TTG compositions. Such anatexis can occur either in a subduction or in a post-subduction environment. This typology, based on the well-exposed Dharwar Craton, can be extended to Late Archaean granitoids from all over the world. About such 500 analysis were compiled, and a number of discriminant diagrams, based on both major and trace elements, are proposed. The most likely tectonic setting for the observed rock types is an accretionary orogen with accretion of continental blocks above a subduction system, followed by thermal reworking of the newly accreted continental material. However, it has been found that Archaean “subduction-related” granitoids are significantly different from their modern counterparts, implying progressive changes in the modes of magma generation at convergent margins from the Archaean to the present.

Spatial and temporal isotopic domains of contrasting igneous suites in Western and Northern Sulawesi, Indonesia

Palaeocene to Pliocene magmatism in NW Sulawesi shows a progression from an Older Series with calc-alkaline/tholeiitic signatures (51–17 Ma) to a Younger Series of mafic-intermediate high-K magmas (∼14–5 Ma) and felsic K-rich calc-alkaline (CAK) magmas (9–2 Ma). The isotopic and geochemical compositions of the Older Series samples indicate that the more westerly samples have been generated in a continental arc setting and the more easterly samples in an oceanic arc; this distinction defines the boundary between the Western and Northern Sulawesi tectonic terranes. The Younger Series high-K magmas have unusual isotopic compositions, with variable but low 143Nd/144Nd, high 87Sr/86Sr values, and high 208Pb/204Pb for their 206Pb/204Pb ratios compared to subduction-related magmas. The diversity of the isotopic compositions points towards a source with a long and heterogeneous geochemical evolution, most likely located within the Australian subcontinental lithospheric mantle. The Younger Series felsic CAK magmatism has a more homogeneous isotopic and geochemical signature and reflects melting of continental crust of Australian origin.This geochemical progression in time is very similar to that seen in central Sulawesi [Chem. Geol. 156 (1999a) 67], and is best explained by normal subduction of an oceanic plate followed by melting of an underthrust sliver of Australian continent. The size of this microcontinent can be estimated from the areal extent of low-Nd-isotope magmas in Western Sulawesi, ranging from approximately 4°S to 1°N. Underthrusting must have happened prior to 14 Ma, indicating that this event cannot be equated to the collision between Sulawesi and the Sula platform, which occurred at 5 Ma.While subduction beneath western Sulawesi ceased prior to the onset of potassic magmatism in this region, it continued beneath northern Sulawesi producing predominantly calc-alkaline suites.

Indian Continental Crust Recovered from Elan Bank, Kerguelen Plateau (ODP Leg 183, Site 1137)

At Site 1137 on Elan Bank of the Kerguelen Plateau, a large igneous province in the southern Indian Ocean, a fluvial, volcaniclastic, polymict conglomerate and a fluvial sandstone are intercalated with subaerially erupted tholeiitic basalt flows. Clasts recovered from the conglomerate have highly variable lithologies, including alkali basalt, rhyolite, trachyte, granitoid and gneiss, Major and trace element abundances and whole-rock isotopic data for the sandstones, the conglomerate matrix and representative clasts from the conglomerate are used to infer the origin of these diverse rock types. The gneiss clasts show an affinity to crustal rocks from India, particularly those of the Eastern Ghats Belt and its possible East Antarctic corollary, the Rayner Complex. The felsic volcanic clasts an not genetically related to the intercalated basalt flows, despite being erupted contemporaneously with these basaltic magmas. These felsic volcanic clasts probably formed from partial melting of evolved upper continental crust. The granitoid also probably formed by partial melting of continental crust and could be an intrusive equivalent of the felsic volcanic rocks. In contrast, the alkali basalt clasts have isotopic compositions that are more similar to those of the tholeiitic basalt flows recovered at Site 1137: however, these clasts are highly alkalic (tephrite to phonotephrite) and have a distinct petrogenesis from the tholeiitic basalt flow units.

Partial melting of mafic (amphibolitic) lower crust by periodic influx of basaltic magma

We present the results of numerical simulations to explore the effects of periodicity of basalt intrusion on the degree and timescales of melting of mafic lower (arc) crust. Melt production as a function of temperature was determined from parameterisation of published (0.7–1.6 GPa) fluid-absent partial melting experiments on amphibolites. We focus on a periodicity ranging from 20 to 30 000 years, with a total of 1 km of basic material intruded. Emplacing new basalt intrusions on top of earlier ones maximises the amount of silicic melt generated in the overlying protolith, and reduces greatly the heat loss through the base of the pile. Except for the case of a single intrusion, average partial melt temperatures generally exceed 900°C, and maximum temperatures may exceed 1000°C. The degree of partial melting is governed by the initial intrusion temperature and the periodicity, and yields a maximum predicted average melt fraction of 0.38. There is a time lag between maximum melt production and the maximum height reached by the melt column, with melt fractions in excess of 0.2 generated only where the time interval between each new intrusion is ≤200 years. Predicted maximum melt thicknesses do not exceed ca. 100 m, implying that large granitic magma chambers may not develop in the source region during partial melting of mafic lower crust. The ratio of the period of intrusion ( τ i) and the characteristic timescale for heat loss ( τ d) defines an important variable R that can be used to assess the thermal behaviour of the melting column, with both melt temperature and average (maximum) melt fraction maximised where R=1. The ratio R is used as an indicator of composition (expressed through changes in REE content) of a hypothetical partial melt, with smaller numbers of thicker intrusions ( R→0) resulting in higher modelled (La/Yb) N ratios. Partial melting of continental crust beneath intrusions may provide a means of mixing mantle and crustal components at source prior to magma emplacement. Alternatively, if crustal melt freezes at the base, strong compositional (mafic–felsic) layering may result. During partial melting by periodic, multiple intrusion, the lower crust is a dynamic environment, with the amount of silicic (granitic) melt available for extraction in the source region rising and falling in the crustal column with time.

Geochemistry and Paleotectonic Setting of Felsic VolcanicRocks in the Finlayson Lake Volcanic-Hosted Massive Sulfide District, Yukon,Canada,

The Finlayson Lake volcanic-hosted massive sulfide (VHMS) district represents one of Canada’s most recent VHMS discovery regions with ~34 million metric tons (Mt) of massive sulfide mineralization found since the mid 1990s. Felsic volcanic rocks are associated with three units: the Fire Lake unit, the Kudz Ze Kayah unit, and the Wolverine succession. Significant accumulations of polymetallic felsic VHMS deposits (Kudz Ze Kayah, GP4F, and Wolverine) have only been discovered in the Kudz Ze Kayah unit and Wolverine succession to date. In the hanging wall of the Money Creek thrust, felsic volcanic and high-level intrusive rocks in the Fire Lake unit have calc-alkalic and tholeiitic affinities with low high field strength element (HFSE) contents and intermediate Zr/Sc (9.4‐43.4) and Zr/TiO2 (254‐864) ratios. These rocks are interlayered with mafic rocks with arc geochemical signatures, to date devoid of significant VHMS mineralization, and represent bimodal magmatism within an evolving Devonian-Mississippian continental-arc system. The Kudz Ze Kayah unit in the footwall of the Money Creek thrust stratigraphically overlies the Fire Lake unit and consists of felsic volcanic rocks with high HFSE contents, within-plate (A-type) signatures, and high Zr/Sc (15.3‐190.3) and Zr/TiO2 (630‐2,185) ratios. The Kudz Ze Kayah unit felsic rocks are crosscut and overlain by alkalic mafic rocks, are associated with abundant carbonaceous sedimentary rocks, and represent magmatism within a Devonian-Mississippian ensialic back-arc rift-basin environment. The Wolverine succession unconformably overlies the Kudz Ze Kayah unit. Felsic rocks below the Wolverine deposit have geochemical attributes similar to the Kudz Ze Kayah unit with high HFSE contents, within-plate (A-type) signatures, and high Zr/Sc (29.9‐84.2) and Zr/TiO2 (391‐1,220) ratios. In contrast, aphyric rhyolite flows in the hanging wall of the deposit have much lower HFSE contents and the lowest Zr/Sc (3.5‐27.7) and Zr/TiO2 (181‐591) ratios in the district. All the felsic rocks of the Wolverine succession are interlayered with abundant carbonaceous sedimentary rocks and are overlain by midocean ridge basalt (MORB)-like basaltic rocks. The Wolverine succession is interpreted to have formed within an Early Mississippian ensialic back-arc basin environment that eventually evolved to sea-floor spreading. The variation in the HFSE budgets of the felsic rocks of the Finlayson Lake district likely reflects variations in the source and/or temperature of crustal melting. In particular, felsic rocks of the Fire Lake unit have higher Nb/Ta and lower Ti/Sc ratios than other volcanic rocks in the district, suggesting possible derivation from mafic crustal sources and/or lower crustal fusion temperatures. The Kudz Ze Kayah unit and footwall rocks to the Wolverine deposit are inferred to have formed from high-temperature partial melting of continental crust. The hanging-wall aphyric rhyolites from the Wolverine deposit may have formed from either lower temperature continental crustal melting or may have been derived from the mixing of HFSE-depleted N-MORB mafic magmas and evolved continental crust. Polymetallic felsic volcanic-associated, VHMS deposits within the Finlayson Lake district are preferentially associated with HFSE-enriched felsic rocks with high Zr/Sc (15.3‐190.3) and Zr/TiO2 (391‐2,105) ratios. The HFSE and rare earth element (REE) systematics of VHMS-associated felsic rocks of the Finlayson Lake district are different from prospective felsic rocks from Archean VHMS environments in the Superior province and are displaced toward higher Zr/Y and La/Ybn ratios. Their HFSE and REE systematics are similar to many Phanerozoic VHMS environments, in particular those at least partially to fully underlain by evolved continental crust. The geochemical differences between the felsic rocks of the Finlayson Lake district and those from Archean VHMS environments most likely reflect differences in the substrates from which the felsic rocks were derived (e.g., evolved versus juvenile).