Intrusion Of Mantle-derived Magmas Research Articles

Geochemistry of the (meta-)mafic rocks from the Gonzalito mining district, northern Patagonia

In spite of hosting one of the most important Pb–Ag–Zn mineralizations in Patagonia, the metamorphic history of the rocks of the Mina Gonzalito Complex (MGC; east of the North Patagonian Massif) is still unclear. The complex consists of schists, para- and ortho-derived gneisses, ranging from greenschist to amphibolite facies, and metamafic rocks. Leucogranites and pegmatites were intruded synkinematically. Field, petrological and thermochronological evidence indicates that the MGC experienced an early prograde path and metamorphic peak during the Early Ordovician (ca. 472 Ma), magmatism and localized post-peak deformation and re-equilibrium at lower pressure, followed by uplift during the Late Permian. The MGC is intruded by the calc-alkaline Santa Rosa Diorite (SiO2 = 58.7–60.4 wt%; LaN/YbN = 7.2–10.5) and trachyte dike swarms in the Late Permian- Early Triassic. The mafic intrusives of the MGC form small schistose, massive and banded bodies interlayered within the gneisses and granites and recorded recrystallization of hornblende + plagioclase + quartz + titanite ± clinopyroxene ± biotite ± ilmenite. The metamafic rocks are mostly tholeiitic gabbros having SiO2 (45.4–52.1 wt%), TiO2 (0.62–2.88 wt%), flat REE patterns (LaN/YbN = 0.48–2.76), although some pyroxene-banded varieties show higher ratios. Initial P–T modelling in the NCKFMASHTO system for the metamafic rocks defined P-T conditions between 550 and 730 °C and 1–4 kbar. Our data suggest that the protolith of the metamafic rocks was emplaced in a shallow environment, associated with underplating of mantle-derived magmas slightly modified by crustal contamination. The intrusion of mantle-derived magmas may have been related either to a magmatic arc or to a continental rift environment. The model involving an Ordovician intracontinental back-arc basin is favored herein because it can reasonably explain many other geological features of Early Paleozoic basement rocks from northern Patagonia.

Ca. 1.04 Ga hot Grenville granites in the western Yangtze Block, southwest China

The late Mesoproterozoic to early Neoproterozoic magmatism in the western Yangtze Block, though minor in volume, has offered a rare opportunity for understanding the role of the Yangtze Block in the Grenville orogeny. The Huidong K-feldspar granites have SIMS zircon U-Pb ages of 1048.5 ± 4.9 Ma and 1043.1 ± 5.1 Ma, whereas the Yuanmou biotite granites were dated at 1041 ± 12 Ma by LA-ICP-MS. Their ages are indistinguishable within error, suggesting that they were emplaced contemporaneously. Both Huidong and Yuanmou rocks show geochemical features typical of A-type granites, including high FeO*/MgO, Ga/Al ratios and elevated contents of high-field-strength elements (HFSE). The Huidong granites exhibit positive whole-rock εNd(t) (+0.58 to +4.4) and zircon εHf(t) (+6.0 to +8.3) values close to those of regional coeval mafic rocks. They have a narrow range of zircon δ18O from 6.2‰ to 7.2‰, which is only slightly higher than the ‘mantle zircon’ value of 5.3 ± 0.6‰. High Nb/La and Nb/Th ratios in these rocks overlap with those of regional coeval mantle-derived rocks. Along with high zircon saturation temperatures of >1000 °C, we suggest that the Huidong granites could have formed mainly by extreme fractionation from basaltic parental magmas, or alternatively by partial melting of newly emplaced tholeiite at high temperatures. By contrast, the Yuanmou granites have lower whole-rock εNd(t) (−2.0 to +0.59) and zircon εHf(t) (−1.5 to +5.1) values and a Hf isotope crustal model age of ∼1.56–1.97 Ga, suggesting that its parental magmas may be generated by partial melting of old crust during intrusion of mantle-derived magmas. Ca. 1.04 Ga granites in the region show geochemical features similar to A-type granites in orogenic belts, e.g., the aluminous A-type granites in the Lachlan Fold Belt, Southeast Australia. They are also characterized by high-extremely high (>1000 °C) temperature in their genesis, similar to coeval hot Grenville granites globally. Temporal correlation of metamorphic and magmatic records between South China and the Grenville Province leads us to conclude that the ∼1.04 Ga A-type granites in the western Yangtze Block form parts of the hot Grenville granites emplaced during post-orogenic crustal extension.

Isotope geochemistry and its implications in the origin of Yangla copper deposit, western Yunnan, China

The Yangla copper deposit is located in western Yunnan Province, China, with an estimated Cu reserve of approximately 1.2 million tons. It is a typical giant copper deposit, and its mining started only recently. The δ13CV–PDB values of the calcites studied vary from −5.1‰ to 1.0‰, implying that the hydrothermal fluids from which the calcites precipitated were derived from the granitic magma. The δ18OSMOW(H2O) and δDSMOW values of quartz fluid inclusions range from 0.11‰ to 2.50‰ and from −120‰ to −100‰, respectively. These data may suggest the following: (1) mixing between meteoric and magmatic fluids, or (2) the evolution of meteoric fluid by its interaction with igneous or metamorphic rocks. The δ34S values of sulfides range from −4.20‰ to 1.85‰ (average: −0.85‰), which is consistent with the magmatic origin. Based on the 3He/4He ratios of fluid inclusions trapped in sulfides of the deposit (0.14–0.17 Ra) and 40Ar/36Ar ratios of 301–1053, it can be inferred that the ore-forming fluids of the deposit were derived primarily from the crust with a minor mantle component during the metallogenic processes. Based on C, H, O, and S isotopic compositions, and the Yangla copper deposit is bordered primarily by gently dipping thrust faults near the Linong granodiorite. Moreover, the 187Re–187Os isochron age of molybdenite puts the time of metallogenesis at 233.3 ± 3 Ma, which is virtually coeval with the emplacement of the Linong granodiorite (235.6–234.1 Ma) and highlights the genetic link between the Yangla copper deposit and the Linong granodiorite. It is likely that the ore-forming fluids exsolved from the Linong granodiorite, which was formed by crustal melting induced by the intrusion of mantle-derived magma. During the late Early Permian, the Jinshajiang oceanic plate was subducted to the west, resulting in the formation of a series of gently dipping thrust faults in the Jinshajiang tectonic belt. Subsequently, the thrust faults was tensional during the early Late Triassic, which was a time of transition from collision-related compression to extension in the Jinshajiang tectonic belt; such conditions produced an environment favorable for the formation of ore fluids. This extension, in turn, induced the upwelling of hot asthenosphere, triggering intense melting in the lithospheric mantle and producing voluminous basaltic magma. Subsequently, the mantle-derived magma likely ascended along the fractures and faults to underplate the lower crust, which underwent partial melting to generate voluminous granitic magma. After the magma reached the base of the early-stage Yangla granodiorite, the platy granodiorite at the base of the Yangla body shielded the late-stage magma. Then, this magma cooled slowly, releasing some of its ore-forming fluids into the gently dipping thrust faults near the Yangla granodiorite and producing mineralization.

Amalgamation of the North China Craton: Key issues and discussion

Geological and geophysical data indicate that the Precambrian basement of the North China Craton (NCC) formed by amalgamation of a number of micro-continental blocks. The number of blocks, when they existed and how they came together are controversial, and in particular the following issues are disputed: (1) the timing of collisional event(s) leading to the amalgamation of the Eastern and Western blocks along the Trans-North China Orogen (TNCO); (2) the polarity of the subduction between the Eastern and Western blocks; (3) the validity of an old continental block (Fuping Block) that collided with the Eastern Block at ∼2.1Ga; (4) the tectonic setting of the northern margin of the NCC in the Paleoproterozoic; (5) the tectonic nature of high-pressure (HP) and ultrahigh temperature (UHT) granulite-facies events in the Khondalite Belt of the Western Block; and (6) the tectonic setting of the Paleoproterozoic Jiao-Liao-Ji Belt in the Eastern Block.Analysis and integration of available stratigraphic, structural, geochemical, metamorphic and geochronologic data enable the development of an internally consistent and coherent model for assembly and stabilization of the various Archean blocks of the NCC in the Paleoproterozoic. All metamorphic ages obtained for the TNCO are around 1.85Ga, which establishes that the final amalgamation of the Western and Eastern blocks of the craton occurred at ∼1.85Ga. The TNCO is characterized by a fan-shaped pattern of structural features, with the top-to-the-NW and top-to-the-SE thrusting in the northwest and southeast, respectively. This pattern does not constrain subduction polarity for the collisional assembly of the Eastern and Western blocks. Structures in lithospheric mantle and asthenosphere in the TNCO have been significantly modified/replaced in the Mesozoic and Cenozoic, and hence the present-day orientation of these structures, even if they relate to Paleoproterozoic assembly of the craton cannot be used to infer associated subduction polarity. There are no unique structural data or available metamorphic data to supporting the existence of an old continental block that intervened between the Eastern and Western Blocks, which collided with the Eastern Block at ∼2.1Ga. Available data are also inconsistent with the existence of the Paleoproterozoic Inner Mongolia-North Hebei Orogen along the northern margin of the NCC that formed through accretion of an exotic arc at ∼2.3Ga and incorporated into the Paleoproterozoic Columbia (Nuna) Supercontinent at 1.92–1.85Ga. We interpret the north Hebei portion of this inferred orogen as part of the TNCO, and the Inner Mongolian portion as an independent continental block (Yinshan Block). This block is separated from the Ordos Block by the Paleoproterozoic Khondalite Belt. The high-/medium-pressure granulite facies metamorphic event in the Khondalite Belt is considered to have resulted from collision between the Yinshan and Ordos blocks to form the Western Block at ∼1.95Ga, whereas the ∼1.92Ga UHT metamorphism within the belt was related to the underplating or intrusion of mantle-derived magmas during the post-collisional extension. The Jiao-Liao-Ji Belt in the Eastern Block likely formed through Paleoproterozoic rifting to form the Longgang and Langrim blocks, and subsequent basin closure and collision in the period 2.2–1.9Ga.

Tectonothermal history of the basement rocks in the western zone of the North China Craton and its tectonic implications

The basement of the North China Craton can be divided into the eastern, central and western zones, based on lithological, structural, metamorphic and geochronological data. The western zone comprises two different petrotectonic units: Archaean tonalitic–trondhjemitic–granodioritic (TTG) grey gneisses and metamorphic mafic rocks, and Palaeoproterozoic khondalite series. The former is characterized by isobaric cooling (IBC)-type anticlockwise P– T paths in the north-northwestern part of the zone and near-isothermal decompression (ITD)-type clockwise P– T paths in the eastern part, adjacent to the central zone. On the other hand, the tectonothermal evolution of Palaeoproterozoic khondalite series rocks is characterized exclusively by nearly isothermal decompression following the peak of metamorphism and then cooling, defining clockwise P– T paths. The Archaean TTG gneisses and associated mafic rocks with anticlockwise metamorphic P– T paths reflects an origin related to underplating and intrusion of mantle-derived magmas which may be derived from mantle plumes. They represent a late Archaean continental block in the western part of the North China Craton. The Palaeoproterozoic khondalite series rocks represent passive continental margin deposits. They were metamorphosed and deformed in the late Palaeoproterozoic during the amalgamation of the western continental block with another continental block in the east part of the North China Craton. The ITD-type clockwise P– T– t paths of the Palaeoproterozoic khondalite series rocks record the tectonothermal histories of the collision of the western and eastern continental blocks which resulted in the final assembly of the North China Craton at c. 1800 Ma.

Thermal evolution of two textural types of mafic granulites in the North China craton: evidence for both mantle plume and collisional tectonics

Mafic granulites from the North China craton can be divided into two textural types, referred to as A- and B-types. A-type mafic granulites display garnet+quartz symplectic coronas, and outcrop in the eastern and western zones of the craton, whereas B-type mafic granulites exhibit orthopyroxene+plagioclase±clinopyroxene symplectites or coronas, and are mainly exposed in the central zone of the craton. Most A-type mafic granulites preserve the prograde (M1), peak (M2) and post-peak near-isobaric cooling (M3) assemblages, which are represented respectively by inclusions of hornblende+plagioclase+quartz, a peak mineralogy of orthopyroxene+clinopyroxene+plagioclase+quartz+garnet, and overprinted by garnet+quartz symplectic coronas. These mineral assemblages and their P–T (pressure-temperature) estimates define anticlockwise P–T evolutionary paths. The B-type mafic granulites preserve the peak (M1), post-peak near-isothermal decompression (M2) and cooling (M3) assemblages, which are represented by the peak assemblage of orthopyroxene+clinopyroxene+plagioclase+quartz+garnet±hornblende, post-peak orthopyroxene+plagioclase±clinopyroxene symplectites or coronas, and later hornblende+plagioclase+magnetite symplectites, respectively. These mineral assemblages and their P–T estimates define clockwise P–T paths.The anticlockwise P–T paths of the A-type mafic granulites in the eastern and western zones of the North China craton are consistent with a model of underplating and intrusion of mantle-derived magmas. In combination with lithological, structural and geochronological data, the eastern and western zones of the North China craton are considered to represent two continental blocks that developed through the interaction of mantle plumes with the lithosphere from the Palaeoarchaean to the Neoarchaean era. The B-type mafic granulites and associated rocks in the central zone represent a magmatic arc that was metamorphosed and deformed during amalgamation of the eastern and western continental blocks in the late Palaeoproterozoic era. The mineral reaction relations and clockwise P–T paths of the B-type mafic granulites from the central zone record the tectonothermal history of the collision that resulted in the final assembly of the North China craton at c. 1800 Ma.

Late Archaean crust-mantle interactions: geochemistry of LREE-enriched mantle derived magmas. Example of the Closepet batholith, southern India

The Closepet batholith in South India is generally considered as a typical crustal granite emplaced 2.5 Ga ago and derived through partial melting of the surrounding Peninsular Gneisses (3.3 to 3.0 Ga). In the field, it appears as a composite batholith made up of at least two groups of intrusions. (a) An early SiO2-poor group (clinopyroxene quartz-monzonite and porphyritic phyritic monzogranite) is located in the central part of the batholith. These rocks display a narrow range in both initial 87Sr/86Sr (0.7017–0.7035) and ɛNd(−0.9to −4.1). (b) A later SiO2-rich group (equigranular grey and pink granites) is located along the interface between the SiO2-poor group and the Peninsular Gneisses. They progressively grade into migmatised Peninsular Gneisses, thus indicating their anatectic derivation. Their isotopic characteristics vary over a wide range (87Sr/86Sr ratios=0.7028–0.7336 and ɛNd values from-2.7 to-8.3, at 2.52 Ga). Field and geochronological evidence shows that the two groups are broadly contemporaneous (2.518–2.513 Ga) and mechanically mixed. This observation is supported by the chemical data that display well defined mixing trends in the ɛSr vs ɛNd and elemental variation diagrams. The continuous chemical variation of the two magmatic bodies is interpreted in terms of interaction and mixing of two unrelated end-members derived from different source regions (enriched peridotitic mantle and Peninsular Gneisses). It is proposed that the intrusion of mantle-derived magmas into mid-crustal levels occurred along a transcurrent shear zone; these magmas supplied additional heat and fluids that initiated anatexis of the surrounding crust. During this event, large-scale mixing occurred between mantle and crustal melts, thus generating the composite Closepet batholith. The mantle-derived magmatism is clearly associated with granulite facies metamorphism 2.51±0.01 Ga ago. Both are interpreted as resulting from a major crustal accretion event, possibly related to mantle plume activity.

Lower crustal rejuvenation and growth during post-thickening collapse: Insights from a crustal cross section through a Variscan metamorphic core complex

The Variscan lower crust of Europe acquired its main features (seismic layering, intrusion of mantle-derived magmas, granulite-grade metamorphism) when the upper crust underwent post-thickening extension through gravitational collapse. We suggest that post-thickening collapse is a geodynamic process that contributes to the growth and differentiation of continental lithosphere in collapsed mountain belts.

Geochemistry and evolution of late Archean plutonism and its significance to the tectonic development of the Slave craton

The intrusion of voluminous Late Archean plutonic rocks is the final event leading to the stabilization of the Slave craton. Three groups of plutonic rocks are recognized in the central part of the province: Group 1 (∼2689–2650 Ma) includes high-Al 2O 3 (Olga Suite) and low-Al 2O 3 (Gondor and Wishbone Suites) trondhjemites and diorite (Central Belt Suite) all of which are temporally or spatially associated with volcanic rocks; Group 2 (∼2610–600 Ma) includes syn- to late deformation hornblende-biotite monzodiorite through granodiorite (Concession Suite) and trondhjemites (Siege Suite); and Group 3 (∼2599–2580 Ma) consists of post-deformational muscovite-biotite granites (Contwoyto Suite) and biotite granites (Yamba Suite). Volcanic and associated mafic plutonic rocks are calc-alkaline and have trace element characteristics diagnostic of rocks formed in modern subduction zone settings. The high-A1 20 3 and low-A1 20 3 trondhjemite suites may be derived by high and low pressure partial melting of mafic crust, respectively. Rocks of the Concession Suite are calc-alkaline, metaluminous and have high contents of Sr, Ba and LREE, high (Ce/Yb) Nratios and negative Nb anomalies. Parental magmas are derived from melting enriched mantle sources and related rocks were produced through assimilation-fractional crystallization and mixing processes in the crust. The younger granites are derived from mixing and homogenization of mantle- and crust-derived materials (Yamba Suite) and the partial melting of metasedimentary protoliths (Contwoyto Suite). Volcanic and Group 1 rocks are interpreted to be remnants of an allochthonous island arc terrane which was accreted to a continental block. The secular evolution in the composition of Group 2 and 3 plutonic rocks reflects a change from dominantly mantle- to crustal-derived magmatism during and following collision. Thickening of the lithosphere during collision, followed by lithospheric detachment caused melting of previously subduction-modified mantle beneath the accreted island arc terranes and generation of magmas parental to the Concession Suite. Intrusion of mantle-derived magmas, in combination with crustal thickening resulted in high T, low P metamorphism, crustal melting and the generation of the two Group 3 plutonic suites.

Collision-related granitoid magmatism and crustal structure of the Hunza Karakoram, North Pakistan

Abstract The Cretaceous to Miocene Karakoram Axial Batholith runs E-W across the middle Hunza Valley in north Pakistan and separates a dominantly sedimentary terrain to the north from a regional metamorphic terrain to the south. A strong phase of deformation and metamorphism associated with the c. 65–50 Ma collision of India with Asia affected the area. The southern part of the batholith and the metamorphic rocks to the south are affected by strong south-vergent thrusting which has brought deeper level, hotter rocks over higher level, cooler rocks so that metamorphic grade increases with structural height. In contrast, the northern part of the batholith is relatively undeformed and shows an intrusive contact with an andalusite-bearing thermal aureole. Two groups of post-collisional magmatic rocks can be recognised in the area. The first group, the Hunza dykes, occurs as a massive dyke complex intruding the early phase of the batholith as well as the sillimanite-bearing gneisses to the south. These dykes consist of a co-genetic suite of granodiorites, monzogranites and leucogranites which appear to be related by feldspar-biotite fractionation. They are volatile-depleted, non-minimum melts with unevolved crustal isotopic ratios and enriched Sr and Ba signatures similar to the Miocene Baltoro pluton further east. They are considered to represent lower-crustal magmas derived by dehydration melting of metasediment, probably promoted by intrusion of mantle-derived magmas. The second group is represented by the Sumayar pluton which intrudes the staurolite-grade metamorphic rocks and is a homogeneous, water-saturated minimum-melt leucogranite, with a major element composition similar to that of the Manaslu leucogranite of central Nepal. Abundant tourmaline and evolved Sr isotope ratios indicate a metapelitic source. Analogy with Manaslu suggests that this source was either fluxed by fluids to promote melting or produced melt under vapour-absent conditions via muscovite breakdown. In the case of the latter, low melt fractions require that an extensive area was tapped to generate a pluton the size of Sumayar. Consideration of the petrogeneses of the two groups of magmas allow constraints to be placed on the deeper crustal structure of the Hunza Valley.

Origins of deep crustal reflections: Implications of coincident seismic refraction and reflection data in Nevada

We compare seismic refraction and reflection results along the PASSCAL/COCORP 40°N transect in the northern Basin and Range of Nevada in order to determine the origin of the prominent reflections from the deep crystalline crust. Reflection data along the transect show a thick zone of discontinuous, subhorizontal reflections, beginning at 4-6 s two-way traveltime (10-20 km depth) and ending at 9-11 s (27-35 km). Two independently derived velocity models, based on refraction data, are largely similar and agree on many important aspects of the reflectivity-velocity relation. Both models show that the top of the reflective zone lies 3-8 km above a prominent mid-crustal velocity discontinuity, which is interpreted to separate bulk silicic from bulk dioritic-gabbroic crust; in most places, the silicic mid-crust is more strongly reflective than the mafic lower crust. This pattern is expected in areas where ductile shearing is the mechanism responsible for the reflectivity. One of the velocity models, however, suggests that, in places, the strongest reflectivity spans both the middle (6.1-6.3 km/s) and lower (6.6 km/s) crust; this pattern suggests that the combined influence of ductile strain fabrics and mafic intrusions gives rise to crustal reflections. Both models show that the lowermost crust and crust/mantle transition are highly reflective, also suggesting the presence of mafic and/or ultramafic intrusions. Thus the observed reflection patterns suggest that ductile shearing and the intrusion of mantle-derived magma—both of which are likely to have accompanied the extreme Cenozoic extension—are important factors in generating deep crustal reflections.

Magnetic measurements over the Hawaiian ridge and their vulcanological implications

The results of an aeromagnetic survey of the Hawaiian Islands are studied in terms of the local magnetic anomalies defined and their relation to known centers of vulcanism. A quantitative analysis as to the depth of origin of the anomalies indicates that in nearly all cases they can be attributed to sub-surface intrusive rock masses at depths of the order of 2 to 6 kilometers. That the intrusive rocks defined are probably related to intrusions of magma derived from the underlying mantle is indicated by both quantitative studies of associated gravity anomalies and seismic refraction crustal measurements. On the basis of these integrated geophysical studies, it appears that there has been extensive intrusion of mantle-derived magma into the crust along the East-West trending submarine Molokai Fracture Zone and that where this fracture system intersected a Northwest-Southeast zone of tectonic weakness or fracture there was vulcanism that led to the formation of the Hawaiian Islands. Although magnetic data are lacking as yet over the entire Hawaiian Ridge from Hawaii to Midway Island, it is not an unreasonable hypothesis that the Ridge as a whole developed in a similar manner over its entire length as a progressive feature with time, in response to a secular shift in crustal stress pattern at the intersection of two major translational fault systems.