Mafic Crustal Materials Research Articles

Silurian to Early Devonian tectonic evolution of the southeastern Qilian Orogen inferred from zircon U–Pb dating and geochemistry of intrusive rocks

Early to mid-Paleozoic tectonothermal events in the Qilian Orogen developed during the closure of the Proto-Tethys Ocean and the convergence of microcontinents in the periphery of East Gondwana. In this paper, we present geochronological, geochemical and Sr–Nd-Hf isotopic data for the granitoid rocks and mafic dykes in southeastern Qilian Orogen. The Liwan (440 Ma), Shixia (434 Ma) and Huchuan (429 Ma) granitoid rocks have metaluminous to weakly peraluminous features, with whole-rock ε Nd ( t ) values of 1.1 to 1.6, −4.2 to −4.4 and −2.6 to 1.1, and zircon ε Hf ( t ) values of 0 to 8.9, −6.6 to 1.6 and −4.6 to 2.2, respectively. Geochemical data suggest that the Liwan, Shixia and Huchuan granitoids are derived from partial melting of the Proterozoic basement with addition of juvenile material, felsic crustal basement and mafic crustal material, respectively. The Zhangjiayuan mafic dykes (403 Ma) are high-K to shoshonitic with whole-rock ε Nd ( t ) (−1.1 to −0.9) and zircon ε Hf ( t ) values (−0.9 to 13.3), derived from an enriched lithospheric mantle. We suggest that these Silurian–Early Devonian intrusive rocks were formed via complex events involving arc-related subduction followed by slab-tearing to post-collisional processes during the Silurian and subsequent lithospheric extension in the Early Devonian. Supplementary material: Tables giving zircon U–Pb isotopic data, whole-rock major and trace element results, whole-rock Sr–Nd isotopic data and zircon in situ Lu–Hf isotopic data are available at https://doi.org/10.6084/m9.figshare.c.6693575

Petrogenesis of Tertiary granitoid rocks from east of the Bidhand fault, Urumieh-Dokhtar Magmatic Arc, Iran: Implication for an active continental margin setting

Tertiary granitoid rocks including diorite, quartz diorite, quartz monzodiorite, monzonite, and quartz monzonite occur in southeast of Qom and to the east of Bidhand fault in the central Urumieh-Dokhtar Magmatic Arc (UDMA). Zircon U–Pb dating reveals an age of ca. 17 Ma for the crystallization of Kerogan quartz diorite. The study samples show sub-alkaline to transitional, metaluminous, and I-type affinity with low to middle SiO2 (53.9–63.9 wt%). Micro granitoid enclaves (MGEs) are widespread in these rocks, exhibiting diorite to monzodiorite composition (SiO2 = 56.75–58.4 wt%). MGEs and host rocks are slightly enriched in LREEs and LILEs, and depleted in HFSE including Nb, Ta and Ti as expected in granitoids formed in an active continental margin setting. The granitoid rocks display initial Sr isotopic compositions of 0.7047–0.7062, positive εNd(t) of 3.2–6.6, and 206Pb/204Pb (18.6431–18.8499), 207Pb/204Pb (15.6322–15. 6902), and 208Pb/204Pb ratios (38.5585–38.9325), and positive zircon εHf(t) values of 6.7–8.7. MGEs also display identical Sr-Nd isotopic compositions (87Sr/86Sri = 0.70537–0.70576; εNd(t) = 2.3–5.3) to the host rocks. The isotopic characteristics in combination with the geochemical signatures indicate that the granitoid rocks and MGEs most likely originated by interaction between mantle-derived mafic and basaltic lower crust-derived melts. Major and trace element modeling reveals that the mafic lower crust is composed of less ~40% partial melting of garnet-free amphibolite. Petrographic and geochemical characterization together with bulk rock Nd-Sr isotopic data suggest that host rocks and associated enclaves derived from the same parent by interaction between basaltic lower crust (~10%) and lithospheric mantle (~90%) during their generation that subduction related compositions have also been involved in their petrogenesis. This magma has experienced high fractional crystallization but minor crustal contamination during magma ascent to the upper crustal level. Petrographical, geochemical, and Sr-Nd-Pb isotopic similarities between granitoid and MGEs imply that MGEs are cognate late cumulates, formed by pressure quenching mechanism during the late stage of magma evolution in crustal magma chamber. The Tertiary magmatism in the study area originated in an arc-related setting induced by slab roll-back of northward Neo-Tethyan oceanic lithosphere subduction beneath central Iranian micro continent and resulted in lithospheric extensional regime and subsequently asthenosphere upwelling. The heat induced by the upwelling of the asthenosphere likely led to melting of upper mantle and the mafic crustal material during Early Miocene as the final magmatism phase in the central part of the UDMA before collision.

Melting of the juvenile lower crust in a far-field response to roll-back of the southern Neotethyan oceanic lithosphere: the Oligocene adakitic dacites, NE Turkey

Late Cenozoic tectono-thermal events and associated magmatism in the Sakarya Zone (SZ) are still contested. Although documented in the western part of the SZ, thus far, no magmatic activity has been identified in the eastern part of the Oligocene SZ. Here, we report a newly identified Oligocene magmatism to interpret the genesis with tectonic setting and gain new insight into the geological evolution of the eastern SZ. We present extensive geochemical, bulk-rock SrNd and zircon Hf isotope, and zircon UPb chronological analyses for the Tepebaşı dacites in the Artvin area, NE Turkey. Zircon UPb dating analyses revealed a dacite formation age of ~29.8 ± 0.3 Ma. Geochemically, with a K2O/Na2O ratio of 0.5 to 0.6, they are composed of rocks of a medium-K calc-alkaline adakitic affinity. The samples are further characterized by low Y (6–7 ppm), and high Sr (362–588 ppm) and Sr/Y ratios (58–98), with low Mg# (41–45) values, demonstrating a close affinity with the crustal source of adakitic rocks. They have slightly radiogenic isotope concentrations (87Sr/86Sr(t) = 0.70460–0.70544, εNd(t) = +1.7 to +2.0), and single-stage Nd model ages of TDM1 = 0.61–0.63 Ga, as well as uniform and positive εHf (t) of 8.2–10.5, with young Hf depleted mantle ages (TDM1 = 0.31–0.41 Ga). These isotopic features, in combination with the geochemical signature, preclude a mantle origin. Instead, they most likely originated from a juvenile mafic lower crustal material by low degree partial melting (<%5) rather than through partial fusion of a subducting slab or thickened lower crust. Trace element modeling reveals that the mafic juvenile lower crust is composed of <10% garnet-bearing amphibolite. Furthermore, trace element compositions imply that adakitic melts formed in an extensional setting without delamination of a thick mafic lower continental crust. We conclude that the Oligocene adakitic magmatism originated in an intracontinental setting, which was subjected to far-field extensional forces induced by roll-back of south Neotethyan oceanic lithosphere just before its detachment in the collision zone. We believe that hot asthenospheric upwelling due to the far-field extension induced by the roll-back of the southern branch of the Neotethyan oceanic lithosphere triggered adakitic magmatism. The heat induced by the upwelling of the asthenosphere likely led to the heat-fluxed melting of juvenile mafic crustal material in such an extensional tectonic setting during the Oligocene epoch in the eastern SZ.

Two stages of granitoid intrusions and their implications on the early Paleoproterozoic tectonic evolution of the Quanji Massif

Tectonic regimes took different forms worldwide during the early Paleoproterozoic, ranging from a slowing or halting of plate tectonics to vigorous subduction-related magmatism. A detailed study is conducted on the Delingha granitoid pluton in the Quanji Massif of northwestern China to reveal the crustal evolution of this Precambrian microcontinent and to provide potential insight on complex and heterogeneous evolutionary processes occurring in this period. The Delingha pluton was mainly composed of gneissic biotite granite and gneissic granodiorite, which formed at ca. 2.48 – 2.44 Ga and 2.38 – 2.35 Ga, respectively, as evidenced by LA-ICP-MS zircon U-Pb ages. The gneissic biotite granite exhibited high-K I-type granite affinities and variable zircon εHf(t) values (−12.9 −4.7) with a major peak of approximately 2.8 and a secondary peak of approximately 3.9, indicating that the source rock was dominated by heterogeneous mafic crustal materials. The gneissic granodiorite shows A-type granite affinities and a nearly unimodal distribution of zircon εHf(t) values (−3.1 −4.0, with a peak of approximately 1.5). These data indicate that the gneissic granodiorite was mainly sourced from Archean mafic igneous rocks. Our study suggests that the gneissic biotite granite formed in a subduction-related environment while the gneissic granodiorite formed in a post-collisional tectonic regime. Alongside previous studies on other Siderian granitoids in the Quanji Massif, we propose a tectonic evolution model, including (1) plate subduction occurring before 2.44 Ga; (2) a collision occurring from ca. 2.43 to ca. 2.40 Ga; (3) a post-collisional extension occurring from ca. 2.39 to ca. 2.32 Ga. This tectonic evolution reveals that the operation of plate tectonics continued during the Siderian and that a slowdown or pause might have occurred in the Quanji Massif after ca. 2.3 Ga.

Lower crustal contribution to the magma formation of the Damiao massif-type anorthosite, North China Craton: Evidence from zircon Hf-O isotopes

The formation of massif-type anorthosites and the associated mangerite, charnockite and (rapakivi-) granite (AMCG suite) are mostly restricted in the “Earth’s Middle Age” (~1.70–0.75 Ga), and the magma sources of massif-type anorthosites are highly debated between the depleted mantle and the mafic lower crust. The ~1.7 Ga Damiao massif-type anorthosite in the North China Craton consists mainly of anorthosite and leuconorite with minor melanorite, mangerite, Fe-Ti-(P) ores and abundant ferrodioritic or gabbroic dykes. In-situ zircon Hf-O isotopic data from anorthosite reveal decoupling of mantle-like δ18O values (5.1–7.4) and ancient crustal-like εHf(T) values (−4.0 to −8.9). Since the significant 177Lu/176Hf reduction in the lower mafic crust, the mafic lower crustal materials derived from depleted mantle could display an ancient crustal medium to high negative εHf(T) values after long period of time (e.g., 0.5–1.0 Ga). In contrast, if no supracrustal materials were added, these mafic crustal materials would retain the mantle δ18O features. Thus, the decoupling of mantle-like δ18O and ancient crustal-like εHf(T) values suggest a lower crustal origin of the Damiao massif-type anorthosite. Compilation of Sr-Nd-O isotopic data of massif-type anorthosites reveal decoupling of εHf(T), initial 87Sr/86Sr and δ18O values, which signifies important lower crustal input in magmas sources of most massif-type anorthosites. We proposed that zircon U-Pb age of the Damiao massif-type anorthosite (~1.70 Ga) and regional age correlation suggest that the anorthosite was emplaced in an intraplate, anorogenic extensional setting after the Paleoproterozoic orogeny had ended.

Episodic crustal growth in the Bundelkhand craton of central India shield: Constraints from petrogenesis of the tonalite–trondhjemite–granodiorite gneisses and K-rich granites of Bundelkhand tectonic zone

Tonalite–trondhjemite–granodiorite gneisses (TTG) and K-rich granites are extensively exposed in the Mesoarchean to Paleoproterozoic Bundelkhand craton of central India. The TTGs rocks are coarse- grained with biotite, plagioclase feldspar, K-feldspar and amphibole as major constituent phases. The major minerals constituting the K-rich granites are K-feldspar, plagioclase feldspar and biotite. They are also medium to coarse grained. Mineral chemical studies show that the amphiboles of TTG are calcic amphibole hastingsite, plagioclase feldspars are mostly of oligoclase composition, K-feldspars are near pure end members and biotites are solid solutions between annite and siderophyllite components. The K-rich granites have biotites of siderophyllite–annite composition similar to those of TTGs, plagioclase feldspars are oligoclase in composition, potassic feldspars have \(\hbox {X}_{\mathrm{K}}\) ranging from 0.97 to 0.99 and are devoid of any amphibole. The tonalite–trondhjemite–granodiorite gneiss samples have high \(\hbox {SiO}_{2}\) (64.17–74.52 wt%), \(\hbox {Na}_{2}\hbox {O}\) (3.11–5.90 wt%), low Mg# (30–47) and HREE contents, with moderate \((\hbox {La/Yb})_{\mathrm{CN}}\) values (14.7–33.50) and Sr/Y ratios (4.85–98.7). These geochemical characteristics suggest formation of the TTG by partial melting of the hydrous basaltic crust at pressures and depths where garnet and amphibole were stable phases in the Paleo-Mesoarchean. The K-rich granite samples show high \(\hbox {SiO}_{2}\) (64.72–76.73 wt%), \(\hbox {K}_{2}\hbox {O}\) (4.31–5.42), low \(\hbox {Na}_{2}\hbox {O}\) (2.75–3.31 wt%), Mg# (24–40) and HREE contents, with moderate to high \((\hbox {La/Yb})_{\mathrm{CN}}\) values (9.26–29.75) and Sr/Y ratios (1.52–24). They differ from their TTG in having elevated concentrations of incompatible elements like K, Zr, Th, and REE. These geochemical features indicate formation of the K-granites by anhydrous partial melting of the Paleo-Mesoarchean TTG or mafic crustal materials in an extensional regime. Combined with previous studies it is interpreted that two stages of continental accretion (at 3.59–3.33 and 3.2–3.0 Ga) and reworking (at 2.5–1.9 Ga) occurred in the Bundelkhand craton from Archaean to Paleoproterozoic.

Middle Jurassic–Early Cretaceous tectonic evolution of the Bayanhushuo area, southern Great Xing’an Range, NE China: constraints from zircon U–Pb geochronological and geochemical data of volcanic and subvolcanic rocks

ABSTRACTThis study presents new zircon U–Pb geochronology, geochemistry, and zircon Hf isotopic data of volcanic and subvolcanic rocks that crop out in the Bayanhushuo area of the southern Great Xing’an Range (GXR) of NE China. These data provide insights into the tectonic evolution of this area during the late Mesozoic and constrain the evolution of the Mongol–Okhotsk Ocean. Combining these new ages with previously published data suggests that the late Mesozoic volcanism occurred in two distinct episodes: Early–Middle Jurassic (176–173 Ma) and Late Jurassic–Early Cretaceous (151–138 Ma). The Early–Middle Jurassic dacite porphyry belongs to high-K calc-alkaline series, showing the features of I-type igneous rock. This unit has zircon εHf(t) values from +4.06 to +11.62 that yield two-stage model ages (TDM2) from 959 to 481 Ma. The geochemistry of the dacite porphyry is indicative of formation in a volcanic arc tectonic setting, and it is derived from a primary magma generated by the partial melting of juvenile mafic crustal material. The Late Jurassic–Early Cretaceous volcanic rocks belong to high-K calc-alkaline or shoshonite series and have A2-type affinities. These volcanics have εHf(t) and TDM2 values from +5.00 to +8.93 and from 879 to 627 Ma, respectively. The geochemistry of these Late Jurassic–Early Cretaceous volcanic rocks is indicative of formation in a post-collisional extensional environment, and they formed from primary magmas generated by the partial melting of juvenile mafic lower crust. The discovery of late Mesozoic volcanic and subvolcanic rocks within the southern GXR indicates that this region was in volcanic arc and extensional tectonic settings during the Early–Middle Jurassic and the Late Jurassic–Early Cretaceous, respectively. This indicates that the Mongol–Okhotsk oceanic plate was undergoing subduction during the Early–Middle Jurassic, and this ocean adjacent to the GXR may have closed by the Late Middle Jurassic–Early Late Jurassic.

The nature of oceanic lower crust and shallow mantle emplaced at low spreading rates

Thermal calculations indicate that at sea floor spreading rates too low to sustain a steady-state magma chamber, most of the upwelling melt still freezes within ∼10 km of the sea floor beneath a mid-ocean ridge. A thickened lower crustal section containing crust-mantle mix is thus predicted. Reduced melting in the source region is an expected effect of low spreading rates. However, cooling from the surface limits the degree to which mafic crustal material is separated from mantle at spreading rates higher than those at which reduced melting becomes important. Evidence for ultramafic contributions to the crust at low spreading rates includes a distinct seismic velocity profile for crust from very slow ridges, evidence of aging of the lower crust, spreading-rate-dependent differences in reflection seismic structure and character, and seismic crustal thicknesses apparently greater than adiabatic crustal thicknesses. Crustal structure inferred from the thermal results agrees with the model of Cannat [Cannat, M., 1993. Emplacement of mantle rocks in the seafloor at mid-ocean ridges. J. Geophys. Res. 98, 4163–4172], which includes a lower crust of mixed composition achieved without the requirement of gross tectonic rearrangement of an originally stratified crustal section.

Crustal structure and the mechanism of extension in the northern Red Sea: Constraints from gravity anomalies

Gravity data from detailed marine surveys were used to investigate the density structure of the crust and upper mantle beneath the northern Red Sea, a continental rift considered by many to be in the process of establishing a mid‐ocean ridge type spreading center. A gravity and bathymetry profile across the sea near 27°N was analyzed in detail. Model constraints include single‐channel and multichannel seismic reflection lines coincident with the gravity profile, which provide detailed information on sedimentary layers, a series of ESP seismic experiments centered nearly on the gravity line which constrain the sediment and crustal thickness, and a series of 58 closely spaced heat flow measurements nearly coincident with the gravity line which closely constrain the mantle thermal structure and thus density variations within the mantle. The calculated gravity anomalies based on the known crustal and mantle structure fail to match the observed data. Specifically, the model anomalies become steadily more negative from the margins toward the center of the sea with the result that the calculated gravity is approximately 25 mGal more negative near the axis relative to the marginal areas than are the observed anomalies. Two possible solutions were investigated. One is to introduce a lower crustal layer resulting from crustal underplating within what was originally considered to be the upper mantle beneath the marginal areas. The other is to introduce an area of slightly denser crust resulting from intrusion of new mafic crustal material within the axial depression. Both solutions can reproduce the observed gravity anomalies within the Red Sea. However, the complete set of geophysical data, and in particular the heat flow in the Red Sea and gravity and seismic refraction data on land, suggests that the most satisfactory model is that the crust of the axial depression is denser than that of the marginal areas. This implies that crustal extension in the northern Red Sea is presently occurring in large part by intrusion of new mafic material.

Volcanic rocks of the Witwatersrand Triad, South Africa. II: Petrogenesis of mafic and felsic rocks of the Dominion Group

A bimodal suite of volcanic rocks builds the bulk of the Dominion Group which, with an age of ∼ 2.72 Ga, is the oldest cover sequence overlying the granite-greenstone Archaean basement of the Kaapvaal craton in the western Transvaal, South Africa. The basic lavas are relatively rich in SiO 2 (50–58%) and aphyric and exhibit a large compositional range. This variation is typically tholeiitic in that it is characterized by strong enrichment of Ti, Fe, and V in differentiated lavas. The different amounts of relative enrichment of these elements allow the basic lavas to be subdivided into two groups. One group is characterized by evolution to low Ti/V ratios in evolved lavas and the other by high Ti/V ratios. For nearly all other elements the composition variation of the two groups is extremely similar and overlaps, although the high-Ti/V group contains the most evolved compositions and most of the primitive lavas (Mg-numbers > 60) belong to the low-Ti/V group. Although recognition of the two groups is based on geochemistry they are interlayered in a consistent pattern within the lava pile. Theoretical modelling indicates that the variation in both lava groups can be explained by low-pressure fractional crystallization of phases in a typical tholeiitic sequence. Primitive basalts of the low-Ti/V group show a large range in major element compositions and a more restricted range of incompatible trace elements and REE abundances. In particular, Fe 2O 3 (total) ranges from 9 to 13.5% and correlates positively with MgO, Cr, and Ni and negatively with Al 2O 5 and SiO 2. There are no correlations of Mg-number with incompatible trace elements such as Zr and Ce. These lavas also have LREE-enriched REE abundance patterns and negative ϵ Nd. These features cannot be explained by crustal contamination but are consistent with derivation by uniform amounts of melting of a mantle source which was heterogeneous with regard to major elements (particularly Fe, Al and probably Ca and Na) and uniformly enriched in incompatible trace elements at least 0.3 Ga before melting. The geochemical style of this enrichment is akin to that associated with modern subduction. This enrichment is associated with the development of a thick Archaean lithosphere, the existence of which is corroborated by diamond age and diamond inclusion thermobarometric studies. The geochemistry of the Dominion basic lavas differs significantly from those of the 3.4 Ga Barberton komatiitic and tholeiitic suites. This suggests that the changing pattern of crustal evolution from metastable ‘greenstone’ tectonic styles to stable ‘Proterozoic’ styles is reflected in the changing character of the mantle sources tapped by basic magmas. The porphyritic Dominion rhyodacites or rhyolites exhibit limited compositional variation and are genetically unrelated to the basic lavas. Nd-Isotopes and La/Th and Th/Co ratios suggest that they are partial melts of mafic crustal materials. Geochemical and petrological features of the Dominion suite are entirely consistent with eruption in a continental environment characterized by rifting or lithospheric thinning. This is consistent with some models for the tectonic development of the Witwatersrand Basin.

Evidence for formation of a flexural backarc basin by compression and crustal thickening in the central Alaska Peninsula

The North Aleutian Basin is a large, Cenozoic sedimentary basin in the northern part of the central Alaska Peninsula and the southern Bering shelf. The gravity anomaly pattern, the geometry, and the structure of the basin suggest that if formed by downward flexure of the backarc lithosphere. Basin modeling suggests that the flexure was driven by the emplacement of surface and subsurface loads having densities comparable to those of oceanic crust and mantle rocks, at approximately the position of the present-day volcanic arc and forearc. The authors suggest that the inferred loads consist of tectonically thickened mafic crustal materials lying beneath the arc and forearc of the central Alaska Peninsula. The crustal thickening may have occurred within a dominantly transpressional regime resulting from oblique convergence between the North American and Pacific plates during the Cenozoic.