Amphibolite Group Research Articles

Arc-generated metavolcanic rocks in the Anshan–Benxi greenstone belt, North China Craton: Constraints from geochemistry and zircon U–Pb–Hf isotopic systematics

The Anshan–Benxi granite-greenstone belt (ABGB) in the Eastern Block of the North China Craton mainly comprises supracrustal rocks of amphibole plagioclase gneisses, plagioclase amphibolites, chlorite schists, biotite schists, quartzites and banded iron formations (BIFs), and intrusive monzogranitic to syenogranitic gneisses. Based on geology, spatial distributions, lithologies and zircon U-Pb geochronology, the metavolcanic rocks of the southern ABGB can be divided into the ∼2527Ma southern plagioclase gneiss group, ∼2536Ma southern amphibolite group, ∼2574 to 2560Ma northern chlorite schist group and northern amphibolite group from south to north. These rocks outcrop as stratified and stratoid beds, whereas some occur as enclaves within later intrusive K-rich granitic gneisses. The southern plagioclase gneiss group exhibits calc-alkaline andesite compositions with high MgO, Cr, and Ni, fractionated REE patterns and depletion in Nb–Ta, which resemble high magnesian andesite (HMA) and are derived from the partial melting of sub-arc mantle peridotite that was previously metasomatized by slab melts. The southern amphibolite group has tholeiitic basalt compositions with slight enrichment in LREEs and high Nb–Ti concentrations but low (Nb/La)PM ratios, which coincide with Nb-enriched basalt and are generated by the remelting of residues of sub-arc mantle after the extraction of HMA melts. The northern schist group shows high-K calc-alkaline andesite-dacite compositions and have highest silica and potassium, and obvious enrichment in LREEs and LILEs and depletion in Nb–Ta, which are comparable to the sanukitoid series rocks. These rocks probably formed by low-degree partial melting of a sub-arc mantle wedge that was metasomatized by slab-derived fluids. The northern amphibolite group exhibits tholeiite compositions with unfractionated REE patterns and low (Nb/La)PM ratios, which are consistent with back-arc basin basalts (BABB) and are produced by partial melting of depleted mantle.Combined with lithological associations of the HMA, NEB, sanukitoid-like rocks and BABB-like rocks, the Anshan–Benxi granite-greenstone belt was produced under a convergent plate environment. Both the zircon εHf values and petrogenesis indicate that the four groups of the metamorphic volcanic rocks directly contributed to continental crustal growth. Integrating with the widespread ∼2.5Ga syenogranites and subordinate K-rich granitoids that were derived from reworking of ancient crust, we propose that the ABGB experienced a significant episode of continental growth dominated by a subduction-related lateral accretion accompanied with contemporaneously strong reworking of ancient crustal materials.

New structural and U–Pb zircon data from Anafi crystalline basement (Cyclades, Greece): constraints on the evolution of a Late Cretaceous magmatic arc in the Internal Hellenides

The Asterousia Crystalline Complex consists of Late Cretaceous amphibolite facies metamorphic rocks and associated granitoids, which can be found in exposures on Crete and the Cyclades (Greece). It is attributed to the Uppermost Unit and therefore to the Pelagonian domain of the Internal Hellenides. The tectonometamorphic evolution of this unit is still a matter of debate. We present new structural and petrological data of Asterousia-type rocks and greenschist facies metamorphic rocks from the island of Anafi in the southern Aegean Sea as well as U–Pb zircon ages of granitoids from Anafi. The crystalline sequence of Anafi rests on top of Eocene flysch and comprises from bottom to top: (a) Anafi Greenschist; (b) Anafi Amphibolite Group (orthoamphibolite with intercalations of metasedimentary rocks at the base); and (c) Chalepa Group (amphibolite facies metasediments with slices of serpentinite and granitoids). LA-ICP-MS and ID-TIMS 206Pb/238U zircon ages of granodiorite from the Chalepa Group reveal several similar zircon populations suggesting continuous emplacement of granitoids inside a magmatic arc from ca. 72.5 to 79 Ma. The minimum emplacement age of granodioritic magma, deduced from the 206Pb/238U median age of the youngest zircon population, is 72.6 +0.1/−0.2 Ma. Deformation (micro)fabrics of granodiorite result from low strain obtained at T > 600 °C. This along with the U–Pb ages and published K–Ar ages indicates intrusion of the plutonic rocks at deep structural levels followed by very slow cooling. Monzogranitic dykes cutting through granodiorite in north-eastern Anafi are undeformed and yielded a 206Pb/238U median age of 69.9 +0.7/−0.7 Ma. Based on the new and published data, the following implications for the tectonometamorphic evolution on Anafi can be made: (1) obduction and accretion of mantle slices (serpentinite) to the Asterousia-type rocks were prior to amphibolite facies metamorphism; (2) intrusion of granitoids during the middle to late Campanian within a magmatic-arc setting and coeval shift of the magmatic arc towards the south; (3) Maastrichtian intrusion of dykes; (4) Palaeocene greenschist facies metamorphism and coeval ductile top-to-the SE thrusting of the Anafi Amphibolite Group on top of the Anafi Greenschist; (5) post-Eocene brittle top-to-the SE thrusting of the Anafi Greenschist and the Anafi Amphibolite Group on top of flysch sediments; (6) clockwise rotation of the system and (Early) Oligocene brittle top-to-the SE thrusting of the Chalepa Group on top of all units mentioned above; and (7) ongoing clockwise rotation of the Aegean block during the Oligocene to Early Miocene and change in stress field (NE–SW compression).

Sm-Nd and Lu-Hf isotope and trace-element systematics of Mesoarchaean amphibolites, inner Ameralik fjord, southern West Greenland

AbstractFragmented supracrustal rocks are typical components of Archaean high-grade gneiss terranes, such as those in the North Atlantic Craton. Here we present the first major, trace element and Nd-Hf isotope data for amphibolites collected in the yet poorly studied southern inner Ameralik fjord region of southern West Greenland. In addition, new U-Pb zircon ages were obtained from the surrounding TTG gneisses.Based on their trace-element patterns, two different groups of amphibolites can be distinguished. Following screening for post-magmatic alteration and outlying ε values, a reduced sample set defines a147Sm/143Nd regression age of 3038 Ma ±310 Ma (MSWD = 9.2) and a176Lu/176Hf regression age of 2867 ±160 Ma (MSWD = 5.5). Initial εNd2970Mavalues of the least-altered amphibolites range from 0.0 to +5.7 and initial εHf2970Ma range from +0.7 to +10.4, indicating significant isotopic heterogeneity of their mantle sources with involvement of depleted domains as well as crustal sources.Surprisingly, the amphibolites which are apparently most evolved and incompatible element-rich have the most depleted Hf-isotope compositions. This apparent paradox may be explained by the sampling of a local mantle source region with ancient previous melt depletion, which was re-enriched by a fluid component during subduction zone volcanism or alternatively by preferential melting of an ancient pyroxenite component in the mantle source of the enriched rocks.

Convergent margin magmatism and crustal evolution during Archean-Proterozoic transition in the Jiaobei terrane: Zircon U–Pb ages, geochemistry, and Nd isotopes of amphibolites and associated grey gneisses in the Jiaodong complex, North China Craton

Mafic metavolcanic rocks and grey gneisses are the dominant components of many Archean terranes and preserve significant imprints and information of the crustal evolution. Here we present in situ zircon U–Pb geochronology, whole-rock geochemistry and Nd isotopes of the amphibolites and associated grey gneisses in the drill holes from the Jiaobei terrane, North China Craton. The LA-ICP-MS zircon U–Pb data show that the magmatic protoliths of two amphibolites and two grey gneisses crystallized at 2457–2503Ma followed by the metamorphism at ∼1.8–1.9Ga. These rocks exhibit heterogeneous whole-rock Nd isotopic compositions, with the most radiogenic analyses (ɛNd(t)=+3.8 to +6.9) plotting on or close to the depleted mantle evolution array and the most unradiogenic ɛNd(t) extending down to −2.4. The data suggest that both ∼2.5Ga depleted mantle and ancient continental crust at least as old as 3.05 Ga contributed to the magma source. In combination with previous zircon Hf isotopic data, it is suggested that the ∼2.5–2.6Ga magmatic events in the entire Jiaobei terrane were mostly derived from the reworking of the pre-existing ∼2.7–2.8Ga crustal material. However, large variations in Hf–Nd isotopic compositions of abundant rocks at ∼2.5Ga and ∼2.7–2.8Ga in the Jiaobei terrane were likely to have resulted from interaction of mantle-derived juvenile magma with ancient crustal materials in the source region. Thus, the peaks of zircon Hf crustal model ages from these rocks do not represent the actual time of crustal growth, but could be artifacts of interaction of magmas from different sources. Geochemical features show that the protoliths of the amphibolites are tholeiitic basalts and can be subdivided into two groups: high-Mg tholeiite basalts (Group I) and high-Fe tholeiite basalts (Group II). Trace element systematics indicate that the Group I amphibolites show good affinity to subduction-related tholeiitic arc basalts, whereas the Group II amphibolites represent back-arc lavas modified by subduction components. The two groups of amphibolites and associated grey gneisses collectively define an arc-back-arc tectonic setting in the study area at ∼2.5Ga. Integrated with the previous data from other areas in Jiaobei, it can be inferred that Archean-Proterozoic transition in the whole Jiaobei terrane witnessed subduction-related magmatism in a convergent margin.

The geodynamic evolution of Mesoarchean anorthosite complexes inferred from the Naajat Kuuat Complex, southern West Greenland

Layered anorthosite complexes are typical components of Archean crustal domains. However, the geodynamic settings in which they were emplaced are still discussed as geological relationships are often ambiguous. Here we report major, trace element and high-precision high-field-strength-element (HFSE) data for the recently discovered well preserved Naajat Kuuat Anorthosite Complex from the inner Ameralik fjord region, southern West Greenland. The dataset is complemented by the first combined Hf–Nd isotope analyses for Archean layered anorthosite complexes and U–Pb zircon geochronology. The data contribute to the small database on Archean layered anorthosite complexes and are used to unravel the origin of these complexes and the tectonic regime involved. Fractional crystallisation of olivine, pyroxene, plagioclase and possibly amphibole controls major and trace element variations in the layered intrusion. There are two groups of amphibolites: (1) a group with primitive mantle normalized trace element patterns are similar to those of MORB-like basalts and (2) typical island-arc tholeiites (IAT), apparently indicating an island-arc setting. Lu–Hf regression lines yield an age of 2985±59Ma (MSWD 4) within the error of the Sm–Nd regression age of 2929±110Ma (MSWD 17). The initial ɛHf(2985) for the Naajat Kuuat rocks range from +1.6 to +5.8 and the initial ɛNd(2985) range from +0.4 to +3.9, either indicating variably depleted mantle sources or variable degrees of crustal contamination. In contrast to most mafic assemblages, ratios of Nb/Ta are highly variable (7.85 to 18.6), reflecting fractionation and accumulation of amphibole, ilmenite and pyroxene. The MORB-like parental liquids have the highest Nb/Ta of ca. 18, consistent with a mantle source overprinted by melt-like components from subducting oceanic crust with high Nb/Ta. The accumulation of plagioclase forming the anorthosites and the primary fractionation of amphibole as well as the occurrence of high-Al basalts within the Naajat Kuuat complex argue for hydrous parental liquids in support of an island-arc related setting. Zircon U–Pb geochronology from the anorthosite and adjacent tonalites reveal major tonalite intrusion into the complex at ca. 2802Ma and a second regional event at ca. 2710Ma, in accord with crustal heating due to micro-continent amalgamation and crustal thickening. Altogether, the geochemical data can be interpreted with a geodynamic model, where anorthosite-complex associated rocks intrude into tectonically thickened island-arc crust. Crustal thickening is possibly triggered by island-arc accretion, leading to the emplacement of TTG bodies that further thickened the crustal pile. Further collision and amalgamation with other proto-crustal assemblages might have led to enhanced crustal magmatism and granulite facies metamorphism.

Geochemistry of the metamorphosed Ordovician Taconian Magmatic Arc, Bronson Hill anticlinorium, western New England

The Bronson Hill anticlinorium (BHA) in western New England is a north- to northeast-trending belt of gneiss domes containing a metamorphosed stratified sequence of Ordovician, Silurian, and Devonian sedimentary and volcanic rocks. These overlie mostly late Ordovician intrusive igneous rocks. The intrusive rocks, exposed in the dome centers, are dominated by felsic gneisses of tonalitic to granitic compositions. They are the exposed parts of a composite batholith that developed along the axis of the Taconian volcanic arc. The Fourmile Gneiss of the Pelham dome, and the Monson Gneiss of the Monson dome, covered in most detail here, are calc-alkaline, dominantly tonalitic to granodioritic gneisses having 61–78% SiO 2. These are divided into two geochemical groups. The most abundant rocks have low Sr (∼50–200 ppm), high Y (∼10–50 ppm), low Al 2O 3, high FeO t, commonly negative Eu anomalies, and have flat to somewhat concave upward MREE and HREE patterns. These rocks were probably derived from melting of plagioclase–pyroxene–amphibole granulites in the deep crust. Rocks of the subordinate high-Sr group are similar in composition to adakites and have high Sr (∼300–600 ppm), low Y (∼1–13 ppm), higher Al 2O 3, lower FeO t, commonly no Eu anomalies, and are strongly depleted in HREE. These rocks were probably derived from a pyroxene–amphibole–garnet ± plagioclase source at higher pressure. Amphibolites are low in abundance in the Fourmile and Monson Gneisses. Most are a low-Nb type (<6 ppm Nb), compositionally similar to typical calc-alkaline island arc basalts. These were probably derived by melting of spinel lherzolite in the mantle wedge under the arc. Gabbroic anorthosite and ultramafic rocks also occur in the Monson dome and appear to be cumulates from magmas similar to the low-Nb amphibolites. A high-Nb (6–12 ppm) amphibolite group was identified and is unique to the Monson Gneiss. These rocks are transitional between calc-alkaline and alkaline basalts, are strongly LREE-enriched (La is 46–440 times chondrite), are enriched in strongly incompatible elements, and two of seven samples have normative nepheline. A broader survey of available analytical data from BHA felsic gneisses shows that gneiss compositions vary from dominantly tonalitic and granodioritic in the southern BHA to dominantly granitic from the Croydon and Mascoma domes (west-central New Hampshire) northward. This variation suggests that the southern part of the arc exposed in the BHA was based on mafic crust, whereas the northern part was based on intermediate to felsic crust. Examination of radiometric dates of Taconian igneous rocks, and biostratigraphic constraints in the Taconic allochthons and in the Taconian foreland, are consistent with a model in which the BHA gneisses are the youngest igneous components of the Taconian arc proper. Collision of this composite arc with Laurentia reached a conclusion in the latest Caradoc or Ashgill of the late Ordovician or in the very earliest Silurian. Younger plutons continuing into the Silurian (e.g., Highlandcroft in part) may be related to magmas generated during post-collision delamination or detachment of the subducted Iapetus oceanic slab.

Archaean rocks from southeastern Karelia (Karelian granite greenstone terrain)

We subdivide southeastern Karelia (Vodlozero) into two different areas on geological grounds. (1) The Vodla River area in which the metamorphic complex contains gneisses and amphibolites which yield UPb zircon ages of 3.14±0.03 Ga and 3.13±0.09 Ga, respectively. They were affected by polyphase deformation and polymetamorphism during 3.1 and 2.8 Ga events. (2) The Lay-ruchey River and Vyg River area where two groups of tonalites with UPb ages of 3.17 and 3.14 Ga can be distinguished by their major and trace element compositions. The first group is similar to some tonalites from the TTG suite in eastern Finland (Martin et al., 1983). The second group is similar to tonalites (TTG suite) from Lapland (Jahn et al., 1984). Two groups of amphibolites can also be distinguished in the Vodlozero region yielding UPb ages of 3.13 and 2.86 Ga. Geological relationships together with geochemical features show the younger group to be comparable to metabasalts from greenstone belts. Tonalites, gneisses and the older amphibolites have model Nd ages which are older than the UPb zircon ages from the same suites.

Geochemistry of the Chakradharpur granite—Gneiss complex—A Precambrian trondhjemite body from West Singhbhum, Eastern India

The Chakradharpur Granite—Gneiss complex (CKPG) is exposed as an elliptical body within the arcuate metamorphic belt sandwiched between the Singhbhum Granite in the south and the Chotonagpur Granite—Gneiss to the north. It consists of an older bimodal suite of grey gneiss and amphibolites, intruded by a younger unit of pegmatitic granite. The bimodal suite represents the basement to the enveloping metasediments. The average major-element chemistry of the grey gneiss conforms to the definition of trondhjemite and includes both low-Al 2O 3 and high-Al 2O 3 types. The amphibolites can be grouped into a low-MgO and a high-MgO type. Rocks of the younger unit range in composition from granodiorite to quartz monzonite. The two granitic units also differ significantly in their Rb, Sr and Ba contents, and in the REE distribution pattern. The grey gneiss shows a highly fractionated REE pattern and a distinct positive Eu anomaly, with Eu/Eu * values increasing with increase in SiO 2 %. In samples of the younger granite, the REE pattern is less fractionated, with higher HREE abundance relative to the grey gneiss and usually shows a negative Eu anomaly. The two types of REE patterns in amphibolites are interpreted to represent the two broad groups identified on the basis of major element chemistry. On the basis of chemical data, a two-stage partial melting model for the genesis of grey gneiss is suggested, involving separation of hornblende and varying amounts of plagioclase in the residual phase. Varying amounts of plagioclase in the residuum result in the wide range of Al 2O 3 content of the partial melt from which the trondhjemites crystallised. Residual hornblende produces the highly fractionated REE pattern, but a relatively higher HREE content of the trondhjemites compared to those produced by separation of garnet in the residual phase. The high level of Ba together with moderate levels of Sr in the trondhjemites can also be explained by plagioclase in the residue, whose effectiveness in partitioning Ba compared to Sr is lower. Of the two groups of amphibolites, the low-MgO type shows relative depletion of LREE compared to the high-MgO type. It contains varying amounts of plagioclase and is tentatively suggested to represent the residue. The other group, with a slightly fractionated REE pattern (Ce N/ Yb N = 2.01), is generally considered to represent the source material for the trondhjemites. This may have been generated by 5–15% partial melting of mantle peridotites, containing higher concentrations of LIL elements than those which produced average Precambrian tholeiites. This first phase of partial melting resulted in the slightly fractionated REE pattern of these amphibolites. Derivation of the younger granitic unit from the trondhjemites can be ruled out on the basis of REE data alone. REE data suggest partial melting of metasediments to be the origin of these rocks. It is also possible that deeply buried volcanic rocks, similar to calc-alkaline components of greenstone belts, are the parent for this component.

Stratigraphical inversion in the Sulitjelma Area, Central Scandinavian Caledonides

SummaryMajor stratigraphical inversions are recognized in two parts of the Sulitjelma area, on the border of Norway and Sweden. In the Otervatn area, inversion is directly demonstrated by way-up structures in pillow lavas of the Sulitjelma Amphibolite Group. The porphyritic amphibolites of the group belong to a penecontemporaneous sheeted intrusive complex which originally underlay the pillow lavas. The other inversion, S of Sorjusjaure, is proposed in order to simplify the stratigraphical succession of the area. The inversions are both thought to belong to the inverted limb of the Gasak Nappe, which is the upper tectonic unit at Sulitjelma. Folding associated with the inversion could cause tectonic repetition of the strata-bound Sulitjelma ore horizon.

The Moine amphibolite suites of central and northern Sutherland, Scotland

SummaryThree groups of amphibolites within the Moine rocks of Sutherland have been analysed for twenty-six elements by XRF analysis. (1) Very rare epidotic hornblende schists within pelite, possibly derived from volcanic ash deposited penecontemporaneously with the pelite (2 analyses). (2) Major and minor bodies of ‘early Moine’ schistose garnetiferous amphibolite intruded early in the deformational sequence (41 analyses). (3) Small bodies of ‘Loch a' Mhòid metadolerite’, intruded later in the deformational sequence (23 analyses). Rare Moine ultrabasites are associated with these metadolerites and probably formed from the same magma as olivine cumulates. These ultrabasites (5 analyses) are chemically distinct from Lewisian ultrabasites (both in the basement sheets within the Moine rocks of Sutherland and in the foreland Lewisian rocks) and from the Glen Urquhart ultrabasic body (3 analyses).Using variation diagrams it is inferred that chemical variation in these amphibolites is due largely to pyroxene and plagioclase crystallization in the early amphibolites and olivine, pyroxene, and plagioclase crystallization in the Loch a' Mhòid metadolerites and ultrabasites, followed in both cases by high-level fractionation. This has led to high values of incompatible elements in many of the early amphibolites. Although geochemical discrimination diagrams tend to give ambiguous results for many of these rocks, the early amphibolites are shown to be tholeiitic in nature with a chemistry transitional between within-plate and island-arc type basaltic magma. The Loch a' Mhòid metadolerites are mildly alkaline within-plate type basaltic magma.