Post-collisional Calc-alkaline Research Articles

Late Neoproterozoic adakitic lavas in the Arabian-Nubian shield, Sinai Peninsula, Egypt

The Sahiya and Khashabi volcano-sedimentary successions are exposed near the southern tip of the Sinai Peninsula, the northernmost segment of the Arabian-Nubian Shield (ANS). These Neoproterozoic successions include a series of intermediate to acidic lavas and associated pyroclastic deposits. Field observations and geochemical data reveal two distinct eruptive phases. The lavas representing each phase are intercalated with volcaniclastic greywackes and siltstones. The first eruptive phase, well exposed at Wadi Sahiya, includes basaltic andesite, andesite and dacite with minor rhyolite. The rocks of this sequence are at most weakly deformed and slightly metamorphosed. The second eruptive phase, well exposed at Wadi Khashabi, includes only undeformed and unmetamorphosed dacite and rhyolite. The two volcano-sedimentary successions were separated and dismembered during intrusion of post-collisional calc-alkaline and alkaline granites.Geochemical compositions of the Sahiya and Khashabi volcanic rocks confirm the field data indicating discrete phases of magmatism, however all the compositions observed might plausibly be derived from a common source and be related to one another dominantly through fractional crystallization. The low and variable Mg# values (55–33) measured in the basaltic andesites and andesites preclude their equilibration with a mantle source. Rather, even the most primitive observed lavas are already the products of significant fractional crystallization, dominated by removal of amphibole and plagioclase. Continued fractionation eventually produced dacite and rhyolite marked by significant depletion in Y and HREE. The gradual appearance of negative Nb-Ta anomalies with increasing SiO2 through both suites suggests at least some component of progressive crustal contamination.The medium- to high-K calc-alkaline character of the Sahiya and Khashabi volcanics could be explained either by their formation at an active continental margin or by a two-stage model that appeals to re-melting of arc material in a post-collisional setting. The Wadi Sahiya basaltic andesite and andesite samples exhibit the defining chemical characteristics of adakites: high Sr (>700 ppm), low Y (<16 ppm), high Sr/Y (>20) and low Yb (<1.8 ppm). Although this signature can be associated with slab melting, here we show that it reflects partial melting of lithospheric mantle beneath thickened continental arc crust. The early eruptive phase, exposed at Sahiya, was erupted on an active continental margin, whereas the later Khashabi succession marks the transition to a post-collisional stage.

Geochemistry, petrogenesis and radioactive mineralization of two coeval Neoproterozoic post-collisional calc-alkaline and alkaline granitoid suites from Sinai, Arabian Nubian Shield

The Younger Granites of Yahmid-Um Adawi area, located in the southeastern part of Sinai Peninsula, comprise two coeval Late Neoproterozoic post-collisional alkaline (hypersolvous alkali-feldspar granites; 608–580 Ma) and calc-alkaline (transsolvous monzo- and syenogranites; 635–590 Ma) suites. The calc-alkaline suite granitoids are magnesian and peraluminous to metaluminous, whereas the alkaline ones are magnesian to ferroan alkaline to slightly metaluminous. Both granitoid suites exhibit many of the typical geochemical features of A-type granites such as enrichment in Nb (>20 ppm), Zr (>250 ppm), Zn (>100 ppm) and Ce (>100 ppm) and high 10000*Ga/Al2O3 ratios (>2.6) and Zr + Nb + Y + Ce (>350 ppm). Accessory mineral saturation thermometers demonstrated former crystallization of apatite at high temperatures prior to zircon and monazite separation from the magma for both granitoid suites. The mild zircon saturation temperatures of the studied Younger Granites (around 800 °C) imply low-temperature crustal fusion and incomplete melting of the largely refractory zircon. The two Younger Granite suites were semi-synchronously evolved during the post-collisional stage of the Arabian-Nubian Shield subsequent to the collision between the juvenile shield crust and the older pre-Neoproterozoic continental blocks of west Gondwana. Their parental magmas has been generated by melting of crustal source rocks with minor involvement from mantle, which might participated chiefly as a source of heat necessary for fusion of the crustal precursor. Extensive in-situ gamma-ray spectrometry revealed anomalously high radioactivity of some Younger Granite exposures along Wadi Um Adawi (eU; 388–746 ppm and eTh; 1857–2527 ppm) and pegmatitic pockets pertaining to the calc-alkaline suite (equivalent U and Th; 212–252 ppm and 750–1757 ppm, respectively). The radioactivity of the syngenetic pegmatites arises from the primary radioactive minerals uranothorite and thorite together with the U- and/or Th-bearing minerals zircon, columbite, samarskite and monazite. The anomalously high radioactivity of some Younger Granite exposures in Wadi Um Adawi stem from their appreciable enclosure of the epigenetic uranium minerals metatorbenite and uranophane.

Genesis of post-collisional calc-alkaline and alkaline granitoids in Qiman Tagh, East Kunlun, China

The post-collisional magmatism of Qiman Tagh is characterized by the intrusion of voluminous intermediate to felsic granitoids, including syenogranite, monzogranite, granodiorite, tonalite and diorite. The granitoids can be divided into two magmatic suites: Calc-alkaline (CA) and alkaline (Alk), which were emplaced from ~236Ma to ~204Ma. The CA suite contains metaluminous granodiorites and monzogranites. Typical Qiman Tagh CA granodiorites show moderately fractionated REE patterns ((La/Yb)N=4.35–25.11) with significant negative Eu anomalies (Eu/Eu*=0.54–1.34), and the primitive mantle-normalized spidergrams show strong depletion of Nb and Sr. The Qiman Tagh CA monzogranites show similar fractionated REE patterns ((La/Yb)N=2.70–13.5) with less prominent negative Eu anomalies, and the chondrite-normalized spidergrams show strongly depleted Ba, Nb and Sr. The Alk suite, including syenogranite, is highly potassic (K2O/Na2O=1.09–3.56) and peraluminous (A/CNK=0.91–1.06). Compared to typical Qiman Tagh CA granodiorites, the Qiman Tagh Alk granitoids can be distinguished by their higher Rb, Nb, Ga/Al, FeO*/MgO, Y/Sr and Rb/Sr, as well as their lower Mg#, MgO, CaO, Al2O3, Sr, Co, V, Eu/Eu*, Ba/Nb, La/Nb, Ba/La and Ce/Nb. The Qiman Tagh CA rocks were most likely to be derived from the partial melting of garnet-amphibolite-facies rocks in the lower crust, leaving behind anhydrous granulite-facies rocks with plagioclase and garnet in the residue. The Alk rocks may have formed by the continued partial melting of granulite-facies rocks at elevated temperatures (>830°C).

Post-collisional magmatism in the northern Arabian-Nubian Shield: The geotectonic evolution of the alkaline suite at Gebel Tarbush area, south Sinai, Egypt

Post-collisional alkaline magmatism (∼610–580 Ma) is widely distributed in the northern part of the Neoproterozoic Arabian-Nubian Shield (ANS), i.e. the northern part of the Egyptian Eastern Desert and Sinai. Alkaline rocks of G. Tarbush constitute the western limb of the Katharina ring complex (∼593 ± 16 Ma) in southern Sinai. This suite commenced with the extrusion of peralkaline volcanics and quartz syenite subvolcanics intruded by syenogranite and alkali feldspar granite. The mineralogy and geochemistry of these rocks indicate an alkaline/peralkaline within-plate affinity. Quartz syenite is relatively enriched in TiO 2, Fe 2O 3, MgO, CaO, Sr, Ba and depleted in SiO 2, Nb, Y, and Rb. The G. Tarbush alkaline suite most likely evolved via fractionation of mainly feldspar and minor mafic phases (hornblende, aegirine) from a common quartz syenite parental magma, which formed via partial melting of middle crustal rocks of ANS juvenile crust. Mantle melts could have provided the heat required for the middle crustal melting. The upper mantle melting was likely promoted by erosional decompression subsequent to lithospheric delamination and crustal uplift during the late-collisional stage of the ANS. Such an explanation could explain the absence or scarce occurrence of mafic and intermediate lithologies in the abundant late- to post-collisional calc-alkaline and alkaline suites in the northern ANS. Moreover, erosion related to crustal uplift during the late-collision stage could account for the lack or infrequent occurrence of older lithologies, i.e. island arc metavolcanics and marginal basin ophiolites, from the northern part of the ANS.

Geochemical and Sr-Nd Isotopic Characteristics of Post-Collisional Calc-Alkaline Volcanics in the Eastern Pontides (NE Turkey)

Major, trace element, K-Ar age and Sr-Nd isotopic data are presented for the Eocene Torul volcanics in the eastern Pontide orogenic belt (NE Turkey). The studied rocks are composed of basaltic andesitic, andesitic, trachyandesitic, and minor trachydacitic lavas associated with their pyroclastics. These rocks contain plagioclase (An2-44), hornblende (Mg#= 0.78-0.98), clinopyroxene (Wo43-46 En41-43 Fs10-15), biotite, quartz, and minor sanidine phenocrysts. K-Ar ages on hornblendes ages range from 43.99 (±2.59) to 33.45 (±2.32) Ma, within the Middle to Late Eocene. The volcanic rocks show calc-alkaline affinities and have medium to high K contents. They are enriched in large ion lithophile (LILE) and light rare earth elements (LREE), with pronounced depletion of high field strength elements (HFSE). The chondrite-normalized REE patterns (Lacn/Lucn= 4.0-9.8) show low to medium enrichment, indicating similar sources for the rock suite. Initial 87Sr/86Sr values vary between 0.70457 and 0.70511 and initial 143Nd/144Nd values between 0.51264 and 0.51278. The main solidification processes involved in the evolution of the volcanics consist of fractional crystallization with minor amounts of crustal contamination ± magma mixing. All evidence supports the conclusion that the parental magma(s) of the rocks probably derived from an enriched upper mantle, previously modified by subduction- induced metasomatism in a post-collisional geodynamic setting.

Nd–Sr–Hf–O isotope provinciality in the northernmost Arabian–Nubian Shield: implications for crustal evolution

Multi-isotope study including whole-rock Nd–Sr, single zircon Hf, and SIMS δ18O analyses of zircons sheds light on magma sources in the northernmost Arabian–Nubian Shield (ANS) during ~820–570 Ma. Reconnaissance initial Nd and Sr isotope data for the older rocks (~820–740 Ma) reaffirms previous estimates that early crustal evolution in this part of the shield involved some crustal contamination by pre-ANS material. Prominent isotope provinciality is displayed by post-collisional calc-alkaline and alkaline igneous rocks of ~635–570 Ma across a NW-SE transect across basement of the Sinai Peninsula (Egypt) and southern Israel. Silicic rocks of the NW-region are characterized by lower εNd(T)–εHf(T) and higher Sri and δ18O compared with rocks of the SE-region, and the transition between the regions is gradual. Within each region isotope ratios are independent of the extent of magma fractionation, and zircon cores and rims yield similar δ18O values. Comparison with southern segments of the ANS shows that the source for most ~635–570 Ma rocks can be modeled as the isotopically aged lower-intermediate crust in the ANS core (SE-region) and its northern, more contaminated ANS margins (NW-region). Nevertheless, Nd–Sr isotope enrichment of the lithospheric mantle is indicated by some basic magmas of the NW-region displaying the most enriched Nd–Sr isotope compositions. Comparison of Nd and Hf depleted mantle model ages for rocks of the SE-region may indicate that crustal formation events in the ANS geographical core took place at 1.1–1.2 Ga and were followed by crustal differentiation starting at ~0.9 Ga.

Post-collisional tectonomagmatic evolution in the northern Arabian–Nubian Shield: time constraints from ion-probe U–Pb dating of zircon

Abstract: Ion-probe U–Pb dating of plutonic rocks from the northern Arabian–Nubian Shield in Sinai and southern Israel constrains the timing of late East African batholithic post-collisional calc-alkaline (CA2) magmatism and within-plate alkaline to peralkaline (AL) magmatism to c . 635–590 Ma and c . 608–580 Ma, respectively. The earliest dated CA2 rocks are slightly deformed to undeformed, indicating that penetrative deformation ceased by c . 630 Ma. Within the CA2 suite a change from mafic to felsic magmatism is manifested in most of the region, peaking in a voluminous pulse of granodiorite to granite intrusion at 610–600 Ma. The AL magmatism started contemporaneously with the peak in CA2 felsic activity at c . 608 Ma and lasted until 580 Ma. It includes mostly alkaline and peralkaline granites, probably representing variable degrees of differentiation of similar parental magmas. Thus CA2 and AL granites do not represent different tectonic settings, but coeval derivation from variable sources during crustal extension. The majority of rocks dated in this study show minor to non-existent zircon inheritance and thus indicate very minor interaction with previously formed felsic crust. The rare zircon xenocrysts span a typical East African age range (900–607 Ma) and confirm the absence of older crustal components in the juvenile Arabian–Nubian Shield. Supplementary material: Geochemical and geochronological methods, sample descriptions and data are available at http://www.geolsoc.org.uk/SUP18327 .

Post-collisional ultrapotassic volcanism in the Tangra Yumco-Xuruco graben, south Tibet: Constraints from geochemistry and Sr–Nd–Pb isotope

Major-trace elements and Sr–Nd–Pb isotopic data are presented for newly discovered felsic ultrapotassic lavas in the Tangra Yumco-Xuruco graben, south Tibet. The felsic ultrapotassic lavas are characterized by high SiO 2 and K 2O, low MgO and CaO contents. They have also high abundances of incompatible trace elements such as large ion lithophile elements (LILEs) and light rare earth elements (LREEs), especially Rb (562–893 ppm), Th (148–182 ppm) and La (101–131 ppm). One of the most striking features of these rocks is their high Th/La (1.15–1.53) and Rb/Sr (1.0–1.26) ratios with pronounced Eu and Ce anomalies (Eu/Eu⁎ = 0.56–0.60, Ce/Ce⁎ = 0.73–0.90), while the high field strength elements (HFSEs) and Sr are depleted relative to LILEs and HREEs. The felsic ultrapotassic lavas show highly radiogenic 87Sr/ 86Sr (0.7200–0.7282) and unradiogenic 143Nd/ 144Nd (0.51191–0.51197), radiogenic 206Pb/ 204Pb (18.794–19.063), 207Pb/ 204Pb (15.697–15.824) and 208Pb/ 204Pb (39.455–39.954). The similarity of trace element patterns and Sr–Nd–Pb isotopic components with the high-MgO ultrapotassic rocks in the province suggests that both the felsic ultrapotassic and mafic ultrapotassic lavas were derived from partial melting of the south Tibetan lithospheric mantle metasomatized by sediment-derived melts. However, the remarkable differences in some key trace element pairs between the felsic ultrapotassic lavas and mafic ultrapotassic rocks in the Tangra Yumco-Xuruco graben indicate that the felsic ultrapotassic lavas were not the highly-differentiated products from the mafic ultrapotassic magmas. Our new data suggest that these felsic ultrapotassic lavas have a distinct magma source beneath the south Tibetan crust, which is different from that of the mafic ultrapotassic lavas. Their high light-REE enrichment (La/Yb ( N) = 41.98–51.41) but low heavy-REE fractionation (Dy/Yb ( N) = 1.99–2.11) suggest an amphibole and phlogopite-rich mantle source in the genesis of the potassic lavas. In contrast, the mafic ultrapotassic lavas in the graben have variable elevated Dy/Yb ( N) (2.1–2.8) and La/Yb ( N) (39–128) ratios, reflecting their sources with residual garnet. By implication, the discrepancy in their source assemblages suggests that the depths of the felsic ultrapotassic sources could be shallower than that of the mafic ultrapotassic sources in the south Tibetan lithosphere. In addition, proposed mantle metasomatism by subducted sediment-derived melts for the genesis of the ultrapotassic lavas might also be applicable for along-strike variation in the post-collisional volcanism in south Tibet. Isotopic mixing calculation illustrates that the subducted sediments represented by the Himalayan basement are an effective reservoir for bulk assimilation of the south Tibetan lithospheric mantle, and hence sediment input of various proportions into the mantle sources could be responsible for the isotopic variation seen in the post-collisional calc-alkaline and potassic volcanics in the eastern part of the southern Tibet. Along-strike variation in the post-collisional volcanism was most probably caused by a westward increase in sediment input in the southern Lhasa block during the subduction of the Tethyan oceanic lithosphere.

Crustal evolution and recycling in a juvenile continent: Oxygen isotope ratio of zircon in the northern Arabian Nubian Shield

Crustal recycling patterns during the evolution of the Neoproterozoic Arabian-Nubian Shield (ANS) were defined using the oxygen isotope ratio of zircon [ δ 18O(Zrn)]. Evidence for early (~ 870–740 Ma) crustal recycling in the northernmost ANS (southern Israel and Sinai, Egypt) is given by laser fluorination analysis of bulk zircon separates, which yield higher than mantle δ 18O(Zrn) values of several island arc complex (IAC) orthogneisses (6.9 to 8.2‰) and also from the average δ 18O(Zrn) value of 6.4‰ determined for detrital zircons (~ 870–780 Ma) from the Elat-schist; the latter representing the oldest known rock sources in the region. These results indicate prolonged availability of surface-derived rocks for burial or subduction, melting, and assimilation at the very early stages of island arc formation in the ANS. Other IAC intrusions of ~ 800 Ma show mantle-like δ 18O(Zrn) values, implying that not all magmas involved supracrustal contribution. Much younger (650–625 Ma) deformed syn-collisional calc-alkaline (CA1) intrusions are characterized by δ 18O(Zrn) values of 5.0 to 7.9‰ indicating continued recycling of the felsic crust. The main sample set of this study comprises rocks from the mostly granitic, post-collisional calc-alkaline (CA2: ~ 635–590 Ma) and alkaline (AL: ~ 608–580 Ma) magmatic suites. Despite having distinct geochemical characteristics and petrogenetic paths and spans of magmatic activity, the two suites are indistinguishable by their average δ 18O(Zrn) values of 5.7 and 5.8‰ pointing to the dominance of mantle-like δ 18O sources in their formation. Nonetheless, grouping the two suites together reveals geographical zoning in δ 18O(Zrn) where a large southeastern region of δ 18O(Zrn) = 4.5 to 5.9‰ is separated from a northwestern belt with δ 18O(Zrn) = 6 to 8‰ by a ‘6‰ line’. It is thus suggested that all CA2 and AL magmas of the northernmost ANS were derived from mantle-like δ 18O reservoirs in the mafic lower-crust and the lithospheric-mantle, respectively. However, while in the northwestern belt these magmas intruded a thick crustal section and assimilated ~ 15–35%, high- δ 18O IAC+CA1 material, magmas in the southeastern region intruded a thinner crust and little or no contamination occurred. The proposed NW–SE variance in crustal thickness during the late Neoproterozoic fits well with the geometry of the fan shaped rifting model proposed by Stern [Stern, R.J., 1985. The Najd Fault System, Saudi Arabia and Egypt: a late Precambrian rift related transform system. Tectonics 4, 497–511.] for this region. Deep parts of the lithosphere were beginning to rift at ~ 630 Ma, allowing the asthenospheric mantle to rise and transfer heat to the lithosphere. This resulted in vast melting of the mafic lower-crust to produce the batholithic CA2 magmas. Later (~ 610 Ma) percolation of lithospheric-mantle melts (possibly along deep seated lithospheric-scale faults) introduced AL magmas to shallow levels of the crust. Intrusion of CA2 and AL mantle-like δ 18O parent magmas into the thinned southeastern crust did not involve assimilation of older crust whereas similar intrusion into the thicker northwestern crust resulted in mild assimilation of high- δ 18O pre-635 Ma crust. An important implication from our results is that petrogenesis of some high- δ 18O AL magmas of the northernmost ANS involved assimilation of supracrustal material. Felsic intrusions of the AL suite were previously described as A-type granites derived solely from mantle melts with no crustal components. Our results contribute to the “A-type petrogenesis debate” by showing that their formation can involve recycling of crustal material.

Petrographic and Geochemical Evidence for Magma Mixing and Crustal Contamination in the Post-Collisional Calc-Alkaline Yozgat Volcanics, Central Anatolia, Turkey

Petrographic, major-oxide, and trace-element data are presented for the Yozgat volcanics. These rocks range in composition from basalts through basaltic andesites and andesites to dacites. Major-oxide variations are largely explicable in terms of fractional crystallization, involving removal of observed phenocrysts and microphenocrysts. However, complex zoning patterns and resorbtion phenomena shown by phenocrysts in these lavas, and observed epitaxitic pyroxene growth around quartz xenocrysts imply that they are hybrids formed by a mixing process. In addition, observed enrichments in crustal elements such as K, Rb, Ba, Sr, and P provide clear evidence for the crustal assimilation of granitoid and metasedimentary xenoliths. The following model is suggested for the evolution of the Yozgat volcanics. The primitive magma underwent fractionation in an intracrustal magma chamber to yield more evolved liquids. Influx of hot, primitive magma into the magma chamber promoted vigorous convection-crustal assimilation and eruption of the volcanic rocks in the study area.