Quartz Dioritic Gneisses Research Articles

Source characteristics of Late Neoarchean diatexite in the Yishan area, western Shandong

鲁西沂山地区高级深熔混合岩中存在丰富的不同时代、不同类型的岩石包体。在详细野外地质工作基础上，本文对高级深熔混合岩中的包体进行了锆石SHRIMP U-Pb定年和Hf同位素分析及全岩地球化学研究，以期探讨沂山地区高级深熔混合岩的源区性质。英云闪长质片麻岩形成时代为~2.7Ga，存在2.69Ga和2.53~2.51Ga两期基性岩浆作用，并记录了2.52~2.50Ga变质作用。角闪斜长片麻岩与黑云角闪变粒岩互层产出，它们可能为变质火山岩，其岩浆锆石年龄为2.51Ga。变质基性岩的岩浆锆石ε_Hf（t）为0~+8.1，一阶段Hf模式年龄为2.83~2.54Ga；变质火山岩的岩浆锆石ε_Hf（t）为-1.2~+5.8，一阶段Hf模式年龄为2.89~2.71Ga。结合前人研究，这套高级深熔混合岩的源区组成包括了新太古代早期的英云闪长质片麻岩和变质辉长岩，新太古代晚期的石英闪长质片麻岩、变质火山岩及斜长角闪岩等岩石类型。~2.5Ga基性岩浆作用在鲁西地区发育广泛，为新太古代晚期深熔作用及高级深熔混合岩和壳源花岗岩的形成提供了热源。;There are abundant enclaves of different ages and types within diatexite in the Yishan area, western Shandong. Based on field work, this paper carried out zircon SHRIMP U-Pb dating and Hf isotope analysis and whole-rock geochemical studies on enclaves in the diatexite. A tonalitic gneiss enclave has a formation age of ~2.7Ga. Two episodes of Neoarchean mafic magmatism are recognized at 2.69Ga and 2.53~2.51Ga, respectively, and they have metamorphic ages of 2.52~2.50Ga. Hornblende-plagioclase gneiss was interbedded with fine-grained biotite-hornblende gneiss, and these two types of gneisses are considered to be meta-volcanic rocks. The hornblende-plagioclase gneiss has a magmatic zircon age of 2.51Ga. The magmatic zircon from the meta-gabbro has ε_Hf(t) values of 0~+8.1 with t_DM1 of 2.83~2.54Ga, whereas the magmatic zircon from the hornblende-plagioclase gneiss has ε_Hf(t) values of -1.2~+5.8 with t_DM1 of 2.89~2.71Ga. Combined with previous work, it is speculated that the source of the diatexite includes Early Neoarchean tonalitic gneiss and meta-gabbro, and Late Neoarchean quartz diorite gneiss, meta-volcanic rock and amphibolite.~2.5Ga mafic magmatism is widely developed in western Shandong, which provides the heat source for the Late Neoarchean anataxis, resulting in the formation of diatexite and crustally-derived granite.

Geochronology, geochemistry and petrogenesis of the Neoarchean magmatism in the Jiefangyingzi area, northern North China Craton: Implications for crustal growth and tectonic affinity

The Bayan Obo-Duolun-Chifeng-Kaiyuan fault is considered as the craton-orogen boundary between the northern North China Craton (NCC) and the Central Asian Orogenic Belt (CAOB). However, locations of this boundary from Duolun, Chifeng to Kaiyuan have not been well constrained. The Jiefangyingzi area northeast to Chifeng is located at the junction of the Bainaimiao arc belt and northern NCC, and has long been regarded as part of the Bainaimiao arc belt of the CAOB. This paper presents new LA-ICP-MS zircon U-Pb geochronological, whole-rocks geochemical and Nd-Hf isotopic data of some recently identified Neoarchean magmatic rocks from the Jiefangyingzi area. These rocks consist mainly of amphibolites, dioritic gneisses, quartz dioritic gneisses, tonalitic gneisses and trondhjemitic gneisses formed at 2569–2536 Ma and minor amphibole plagioclase gneisses formed at ca. 2640 Ma, and are distributed in an east–west belt that is about 13 km long and 1.5 km wide. Geochemical and Nd-Hf isotopic compositions suggest that the amphibolites were derived from partial melting of a depleted mantle that was metasomatized by subduction-derived fluids with significant crustal contamination, and the dioritic gneisses were generated by partial melting of mantle peridotite metasomatized by slab-melts. The quartz dioritic and tonalitic gneisses originated from high degree partial melting of the subducted oceanic crust with garnet-bearing amphibolite facies in the residue, while the trondhjemitic gneisses originated from low degree partial melting of the thickened mafic lower crust with eclogite facies in the residue due to underplating of mafic magmas. The occurrences of Neoarchean basement rocks, combined with previous Nd-Hf isotopic data of the late Paleozoic magmatic rocks, suggest that the Jiefangyingzi area has a tectonic affinity with the Eastern Block of the NCC rather than the CAOB, and the craton-orogen boundary between the northern NCC and the CAOB should be situated in places north to the Jiefangyingzi area. Our results support that the late Neoarchean (2.6–2.5 Ga) is also a period of crustal growth as well as extensive reworking of the 2.9–2.7 Ga ancient crust in the northwestern margin of the Eastern Block of the NCC. In addition, the transition from tonalite-trondhjemite-granodiorite (TTG) and other potassium-poor granitoid gneisses to crustal-derived potassium-rich granitoid gneisses at the end of Neoarchean (ca. 2.53 Ga) marked the initial cratonization of the NCC and ca. 2.53 Ga is a critical transformation period of maturation of the continental crust in the NCC.

The geology of the Clifden district, Connemara Co. Galway, Ireland and present understanding of Connemara geology

The Clifden district, illustrated with an accompanying new detailed 1:12,500 geological map and fold trace map, includes most of the stratigraphy of the Connemara Dalradian and is used to summarise present understanding of the older geological history of Connemara in the light of new structural work and recent age determinations. The Grampian metamorphism peaked with sillimanite formation during and after the late D2 syntectonic intrusions of metagabbros at 475–470Ma and the formation of the major D2 Derryclare fold which was then repeatedly folded by numerous major D3 folds synchronous with the injection in the south of the 467Ma Quartz diorite gneiss suite, maintaining the high-grade metamorphism. Late D3 saw the general initiation of cooling and uplift, the latter pronounced in eastern Connemara above and around the subterranean gathering of the Oughterard Granite magma, plus movement on the late D3 Renvyle-Bofin slide, generating an area of lower pressure metamorphism with cordierite and andalusite not found in the Clifden area.

The geology of the Clifden district, Connemara Co. Galway, Ireland and present understanding of Connemara geology

The Clifden district, illustrated with an accompanying new detailed 1:12,500 geological map and fold trace map, includes most of the stratigraphy of the Connemara Dalradian and is used to summarise present understanding of the older geological history of Connemara in the light of new structural work and recent age determinations. The Grampian metamorphism peaked with sillimanite formation during and after the late D2 syntectonic intrusions of metagabbros at 475–470Ma and the formation of the major D2 Derryclare fold which was then repeatedly folded by numerous major D3 folds synchronous with the injection in the south of the 467Ma Quartz diorite gneiss suite, maintaining the high-grade metamorphism. Late D3 saw the general initiation of cooling and uplift, the latter pronounced in eastern Connemara above and around the subterranean gathering of the Oughterard Granite magma, plus movement on the late D3 Renvyle-Bofin slide, generating an area of lower pressure metamorphism with cordierite and andalusite not found in the Clifden area.

Synchronous late Neoarchean Na- and K-rich granitoid magmatism at an active continental margin in the Eastern Liaoning Province of North China Craton

The appearance of voluminous K-rich granitoid rocks during Late Archean marks gradual maturation and stabilization of the continental crust. While most K-rich granitoid magmatism followed TTG magmatism, they occur synchronously sometimes, and this relationship is crucial for our understanding of the evolution of late Archean continental crust and its geodynamic setting.In this study, a series of ~2.57–2.52 Ga coeval and diverse granitoid rocks, including quartz dioritic-trondhjemitic and granodioritic to monzo-/syenogranitic gneisses, were identified in the southern Fushun area of Eastern Liaoning Province, North China Craton. The ~2.57 Ga quartz dioritic gneisses show moderate MgO (≤ 3.96 wt.%) and moderate to high Mg# (37.3–75.5). Geochemical modeling, together with mildly fractionated REE patterns and negative Eu anomalies and depleted zircon ƐHf(t2) (+2.1 − +7.4), suggest that they were differentiated from a depleted mantle source that was metasomatized by slab-derived fluids. The younger trondhjemitic gneisses (~2.55–2.52 Ga) are divided into two subgroups, i.e., an older subgroup (~2547–2540 Ma) characterized by mildly fractionated REE patterns and negative Eu anomalies, and a younger subgroup (~2533–2517 Ma) with strongly fractionated REE patterns and positive Eu anomalies. The trondhjemitic gneisses lack evidence for magma differentiation, and they are inferred to have been formed by reworking of amphibolites/greywackes at diverse crustal levels, with some inputs of mantle materials in the earlier subgroup. The coeval ~2550–2529 Ma granodioritic and monzo-/syenogranitic gneisses are characterized by high K2O/Na2O (0.67–2.45) but low MgO and Mg# (mostly <2 wt.% and < 50, respectively). Geochemical modeling data indicate that the granodioritic gneisses are less differentiated, which could have been derived from the partial melting of high-K mafic rocks (e.g., regional late Neoarchean calc-alkaline meta-basaltic rocks), as further supported by the constant A/CNK (1.00–1.14) and zircon ƐHf(t2) (+2.0 − +9.3) values. In comparison, the monzo-/syenogranitic gneisses show variable A/CNK (0.78–1.32) and zircon ƐHf(t2) (−2.4 − +7.8) values. They are explained to be formed by the partial melting of mixed sources of high-K mafic and sedimentary rocks, with the primary magmas showing plagioclase and apatite fractionation.Taken together, the above granitoid rocks record gradually decreasing zircon ƐHf(t2) values and increasing crustal thickness. Considering the petrogenetic information, regional geological data and the presence of some ~3.45–2.70 Ga crustal materials, the late Neoarchean granitoid magmatism of southern Fushun were likely generated via complex crust-mantle interaction processes at an active continental margin. It is further emphasized that Archean active continental margins are key sites for the initial maturation of the crust, and this tectonic scenario acted as a trigger for granitoid diversification during both the subduction and subsequent collisional stages.

Crust-mantle geodynamic origin of ~2.7 Ga granitoid diversification in the Jiaobei terrane, North China Craton

The crust-mantle geodynamic origin of global late Archean granitoid diversification remains enigmatic. A series of early Neoarchean diversified granitoids were newly identified in the Jiaobei terrane of the North China Craton, including ~2749–2739 Ma Na-rich quartz dioritic, tonalitic, and trondhjemitic (DTTG) gneisses, and ~2718–2701 Ma K-rich quartz monzodioritic, granodioritic, and monzogranitic gneisses.The quartz dioritic and TTG gneisses commonly show transitional field relationships. They have low K2O/Na2O (0.19–0.59), and are characterized by decreasing MgO, FeOT, TiO2, P2O5, and CaO, changing Eu anomalies from negative to positive, and more fractionated REEs, with increasing SiO2. Both the quartz dioritic and TTG gneisses possess depleted zircon εHf(t2) values (+1.4 to + 7.0). Through quantitative petrogenetic modeling, a fractional crystallization model is invoked, i.e., the quartz dioritic gneisses were derived from partial melting of a depleted mantle source metasomatized by slab-derived melts, whereas TTG gneisses (a low silica and a high silica group) formed by different degrees of fractionation from the quartz dioritic magmas, with inferred liquidus minerals of hornblende, clinopyroxene, plagioclase, and apatite. In comparison, the K-rich granitoids show higher K2O/Na2O (0.69–2.87), with some containing euhedral muscovites. The quartz monzodioritic and granodioritic gneisses have higher MgO and Mg#, but lower Fe* (FeOT/(FeOT + MgO)) than the monzogranitic gneisses. The zircon εHf(t2) values (-0.5 to + 5.6) are lower than those of DTTG gneisses. It is suggested that these K-rich granitoids formed by partial melting of either a metasomatized mantle source (with recycled sediments) or metasedimentary rocks at diverse crustal levels.Early Neoarchean crust-mantle interactions of the Jiaobei terrane are reconstructed, including (1) earlier slab-mantle wedge interactions under a continental marginal arc, forming quartz dioritic and the derivative tonalitic and trondhjemitic rocks; (2) ongoing convergence led to high relief of the arc mountain, facilitating erosion and reworking/recycling of sediments; and (3) partial melting of coupled crust-mantle system following accretion between the above arc system and regional ~2.9 Ga or older continental nucleus generated different K-rich granitoids. It is emphasized that the construction and erosion of thickened arcs are prone to the formation of both high pressure TTGs and K-rich granitoids, leading to the late Archean granitoid diversification globally.

New light on the geology of the Roundstone intrusion, its inversion and that of the Grampian metagabbro-gneiss complex, Connemara, western Ireland

The northern part of the ~470 Ma syntectonic D2 basic and ultrabasic Roundstone intrusion into the Connemara Dalradian metasediments has been mapped in detail for the first time and consists of metagabbros containing a 180m peridotite xenolith and later injections of Quartz diorite gneiss which silicified and agmatised some of the metagabbros. With the already studied southern (Errisbeg) part, plus over 300 chemical analyses of rocks and over 400 probe analyses of minerals, this enables a synthesis of the geology of the whole intrusion. A tholeiitic magma crystallised olivine Fo83-76, orthopyroxene En84-59, diopside to salite and plagioclase from An96-42, and late stage cumulus magnetite. The exceptionally high An% is indicative of high pH2O with copious water (derived from dewatering of a subduction zone?) as is the abundant hornblende that replaced most of the magmatic minerals, the magma being comparable with that of the Lesser Antilles Island arc. The intrusion was formed by numerous already fractionated magma pulses intruded in upward sequence, closely (~467 Ma) followed by pulses of Quartz diorite to granite gneisses with magmatic hornblende, not by closed system fractionation of a single magma injection. The peridotites were mostly carried in as already solidified xenoliths by pulses of gabbro magma. Only the larger bodies in the Errisbeg part of the intrusion were intruded as magma.

New light on the geology of the Roundstone intrusion, its inversion and that of the Grampian metagabbro-gneiss complex, Connemara, western Ireland

The northern part of the ~470 Ma syntectonic D2 basic and ultrabasic Roundstone intrusion into the Connemara Dalradian metasediments has been mapped in detail for the first time and consists of metagabbros containing a 180m peridotite xenolith and later injections of Quartz diorite gneiss which silicified and agmatised some of the metagabbros. With the already studied southern (Errisbeg) part, plus over 300 chemical analyses of rocks and over 400 probe analyses of minerals, this enables a synthesis of the geology of the whole intrusion. A tholeiitic magma crystallised olivine Fo83-76, orthopyroxene En84-59, diopside to salite and plagioclase from An96-42, and late stage cumulus magnetite. The exceptionally high An% is indicative of high pH2O with copious water (derived from dewatering of a subduction zone?) as is the abundant hornblende that replaced most of the magmatic minerals, the magma being comparable with that of the Lesser Antilles Island arc. The intrusion was formed by numerous already fractionated magma pulses intruded in upward sequence, closely (~467 Ma) followed by pulses of Quartz diorite to granite gneisses with magmatic hornblende, not by closed system fractionation of a single magma injection. The peridotites were mostly carried in as already solidified xenoliths by pulses of gabbro magma. Only the larger bodies in the Errisbeg part of the intrusion were intruded as magma.

Effects of thermal annealing and chemical abrasion on ca. 3.5 Ga metamict zircon and evidence for natural reverse discordance: Insights for U[sbnd]Pb LA-ICP-MS dating

We present a microstructural and UPb systematics study comparing pristine, thermally annealed (TA) and chemically abraded (CA) ~3500Ma zircon from a quartz-dioritic gneiss, with the aim to improve pre-analytical workflows for more accurate and precise LA-ICP-MS UPb dating of ancient zircon. Four zircon domains are identified: i) low- to medium-U concentric-oscillatory zoned cores, ii-iii) two porous high-U outer alteration domains, and iv) low-U narrow inward growing recrystallization rims. Raman spectroscopy on the pristine zircon reveals a positive correlation between increasing structural damage and U content. The porous high-U outer domains show a drastic increase in non-formula Ca above estimated amorphous fractions of ~0.8, which we ascribe to hydrothermal alteration of high-damage zircon characterized by percolating networks of amorphous areas. Upon treatment (TA, CA), hyper-spectral CL and Raman spectroscopy suggest structural recovery of point defects, but not full repair of high-damage amorphous areas. Calculated Raman radiation damage ages suggest that natural annealing affected all domains at ~500Ma, consistent with negligible laser ablation matrix effects comparing pristine and treated cores.We show that reverse discordance in pristine and TA cores is not an analytical artifact. High-U alteration domains are normal discordant and were partially reset at ca. 3410Ma. Upon CA, UPb discordance and scatter are reduced in the cores, yielding intercepts of 3503±14Ma and 560±550Ma (MSWD=1.0). The lower intercept matches the timing of Pb-loss recorded in the alteration domains and the Raman radiation damage age. We argue that short distance Pb redistribution within the cores, which led to reverse discordance, was controlled by UPb systematics recording both the crystallization age and the timing of Pb redistribution. Some excess Pb was redistributed from the partially re-set alteration domains into the cores, leading to mixed UPb systematics and further reverse discordance. Removal of the latter excess Pb upon CA suggests that radiogenic Pb from the alteration domains accumulated within distinct damage sites accessible for CA leaching.We conclude that standardization of pre-analytical treatment is not recommended; we propose to investigate the structural state of unknown specimen by means of Raman spectroscopy, in combination with commonly applied CL imaging. During LA-ICP-MS analyses, co-measurement of non-formula and selected trace elements is deemed highly valuable in detecting ablations for use in reliable UPb age determination.

Neoproterozoic eclogite- to high-pressure granulite-facies metamorphism in the Mozambique belt of east-central Tanzania: A petrological, geochemical and geochronological approach

This study investigated Neoproterozoic (Pan-African) eclogite- and high-pressure-granulite (E-HPG) facies rocks from the Mozambique belt of east-central Tanzania, collected close to the town of Ifakara and the adjacent Furua area from different tectonic settings, the Palaeoproterozoic Usagaran and the Neoproterozoic Mozambique belt. The studied rocks are E-HPG facies granite- and diorite-gneisses and a meta-gabbroic rock, which are retrogressed to amphibolite- and greenschist-facies conditions. Four different clockwise P-T paths were constructed. The first P-T path for a granodioritic gneiss displays peak metamorphic conditions at ~830°C and ~13.0kbar. The second P-T path for a quartz dioritic gneiss shows peak metamorphic conditions of ~920°C and ~14.9kbar. The third P-T path for a mafic granulite shows peak metamorphic conditions of ~820°C and ~13.2kbar. A fourth P-T path for a monzodioritic gneiss also displays peak metamorphic conditions of up to ~810°C and ~14.9kbar. Evidence for all four P-T paths is provided by mineral chemical and modal abundance calculations in combination with textural observations in thin sections. Zircon ages indicate that the east-central part of the Mozambique belt in Tanzania consists of granite-, granodiorite- and monzodiorite gneisses with Mesoarchaean (~2915Ma), Neoarchaean (~2637–2676Ma) and Palaeoproterozoic (~1873–1926Ma) protolith ages. Early Neoproterozoic (Tonian) igneous zircons were found in the mafic granulite with an age of ~989Ma. Late Neoproterozoic (Cyrogenian) igneous zircons were found in a dioritic and monzodiorite gneiss with ages of ~748Ma and ~718Ma, respectively. Metamorphic zircons extracted from Qtz-monzodiorite and granodiorite gneisses yielded ages of ~640Ma and are considered to approximate the peak of regional E-HPG metamorphism. We suggest that this high-grade metamorphic event was caused by the collision of fragments of East and West Gondwana during the Pan-African orogeny, associated with ocean closure and the formation of Andean-type continental arc domains, which ended in the formation of a collisional belt.

Neoarchean DTTG gneisses in southern Liaoning Province and their constraints on crustal growth and the nature of the Liao-Ji Belt in the Eastern Block

The Neoarchean metamorphic basement block (NBB) in the southern Liaoning Province (SLP) is located to the southeast of the Liao-Ji Belt and is an important part of the Eastern Block of the North China Craton (NCC). The NBB consists mainly of quartz diorite-tonalite-trondhjemite-granodiorite (DTTG) gneisses. Based on the petrological and chemical compositions, the DTTG gneisses are divided into two types: quartz dioritic gneisses and tonalite-trondhjemite-granodiorite (TTG) gneisses. LA-ICPMS zircon U-Pb isotopic dating reveals that the magmatic precursors of the TTG gneiss samples emplaced at 2548Ma, and that the magmatic precursors of the quartz dioritic gneiss samples emplaced at 2551–2512Ma, except for sample 15DL10-1 from Liangjiadian, which was formed earlier at 2669Ma. Petrogenetic studies indicate that the TTG magmas were formed by partial melting of the juvenile lower crust and that the magmatic precursors of the quartz dioritic gneiss samples were formed by the mixing of magmas derived from partial melting of both the depleted mantle and the juvenile lower crust. Combined with previous chronological data, our newly obtained zircon U-Pb dating data suggest that the NBB in the SLP experienced magmatism at∼2.68–2.63Ga and∼2.56–2.51Ga and metamorphism at∼2.50–2.47Ga. Integrating the zircon U-Pb chronological data with Lu-Hf isotopic data from the DTTG gneisses, we suggest that the NBB in the SLP was subjected to two major episodes of crustal growth at∼2.7Ga and∼2.5Ga. These two episodes of crustal growth at∼2.7Ga and∼2.5Ga are extensive in the Eastern Block (EB) of the NCC. Based on the Archean geological events in the two flanks of the Paleoproterozoic Liao-Ji Belt (LJB), a unified late Neoarchean crystalline basement is proposed in the east of the EB prior to the LJB, which indicates that the LJB was a Paleoproterozoic continental rift belt.

Neoarchean arc magmatism and crustal growth in the north-eastern North China Craton: Evidence from granitoid gneisses in the Southern Jilin Province

Late Neoarchean granitoid gneisses are widespread in the Jiapigou–Jingyu area of the Southern Jilin Province (SJP) of the North China Craton (NCC). These rocks are composed of tonalitic–trondhjemitic–granodioritic–dioritic (TTGD) gneisses and preserve important information on the geodynamic evolution and Archean crustal growth along the northeastern margin of the NCC. Here we present LA–ICP–MS zircon U–Pb isotopic ages which reveal that the magmatic precursors of the tonalitic–trondhjemitic gneisses were emplaced at ∼2578–2574Ma, and the granodioritic–quartz dioritic magmas crystallized at ∼2546–2524Ma. The rocks were subjected to regional amphibolite facies metamorphism at ∼2510–2490Ma.Geochemically, the tonalitic–trondhjemitic gneisses can be subdivided into two subgroups. The magmas of high-Yb subgroup were derived by partial melting of dominantly ancient basaltic materials at the crustal level in the stable field of amphibole, clinopyroxene and significant amounts of garnet, and subsequently subjected to varying degrees of fractionation. Magmas of the low-Yb subgroup were generated by partial melting of dominantly juvenile basaltic materials with minor greywackes at the lower crustal level in the stability field of garnet and rutile, with a possible eclogitic residue. The magmatic precursors of granodioritic gneisses were produced by partial melting of subducted oceanic slabs and minor sediments, with the melts contaminated by the mantle wedge materials during their ascent. The parental magmas for the quartz dioritic gneisses were possibly generated by partial melting of a depleted mantle source that was significantly metasomatized by slab-derived fluids or melts.Combined with their petrogenesis, ∼2578–2574Ma tonalitic–trondhjemitic gneisses display positive zircon εHf(t2) values (+0.4 to +6.2) with TDM(Hf) ages of 2856–2643Ma, suggesting a crustal reworking process. On the other hand, ∼2546–2524Ma granodioritic to quartz dioritic gneisses have highly depleted zircon εHf(t2) values (+4.5 to +7.7) and TDM(Hf) ages of 2682–2550Ma that are close to their magmatic crystallization ages, indicating a significant crustal growth. Integrated with our recent studies on ∼2683–2654Ma metavolcanic rocks in Helong granite-greenstone belt and voluminous ∼2588–2536Ma metavolcanics in Jiapigou granite-greenstone belt, we infer that the Southern Jilin Province experienced Neoarchean subduction-related evolution history, with an Andean-type active continental margin along the northern margin of the NCC, and witnessed two major episodes of subduction-related crustal growth at ∼2.7Ga and ∼2.6–2.5Ga, with the latter period coupled with reworking of old crustal materials.

Petrogenesis of taxitic dioritic–tonalitic gneisses and Neoarchean crustal growth in Eastern Hebei, North China Craton

The taxitic dioritic–tonalitic gneisses in the Zunhua–Qinglong microblock of Eastern Hebei, North China Craton, are composed of dioritic, quartz dioritic, tonalitic and trondhjemitic gneisses. They show taxitic or banded structures and were subjected to amphibolite- to local granulite-facies metamorphism. Based on their whole rock geochemistry, the dioritic and quartz dioritic gneisses (DQ Group) can be classified into a high Ni Subgroup (HNDQ Subgroup) and a low Ni Subgroup (LNDQ Subgroup), and the tonalitic and trondhjemitic gneisses (TT Group) into a low-fractionated Subgroup (LFTT Subgroup) and a high-fractionated Subgroup (HFTT Subgroup). LA-ICP-MS zircon U–Pb isotopic dating reveals that the magmatic precursors of the HNDQ, LNDQ and LFTT subgroups were emplaced from 2522±13 to 2533±14Ma, whereas those of the HFTT Subgroup emplaced at 2499±4Ma. Studies in petrogenesis revealed that the magmatic precursors of the HNDQ Subgroup were derived from partial melting of mantle peridotites metasomatized chiefly by slab melts, while those of the LFTT Subgroup were formed by fractional crystallization of the magmas of the HNDQ Subgroup, with clinopyroxene, magnetite, plagioclase and apatite as the major fractionated phases. Magmatic precursors of the LNDQ Subgroup were derived from partial melting of mantle peridotites metasomatized in a dominant manner by slab-derived fluids, with melts experiencing fractionation of hornblende, magnetite, plagioclase and apatite during the magmatic evolution. Further, magmatic precursors of the HFTT Subgroup were derived from partial melting of volcanic sedimentary rocks formed in the fore arc to trench belt, which were subducted along with oceanic slabs, with the melts contaminated by mantle peridotites during their ascent through the mantle wedge.These new data, combined with previous investigations on metavolcanic rocks and granitoid orthogneisses, suggest that the magmatic precursors of these taxitic dioritic–tonalitic gneisses were formed in an arc to back-arc tectonic setting from north to south. Furthermore, Lu–Hf isotopic data from the late Neoarchean metavolcanic rocks and granitoid gneisses in the Zunhua–Qinglong microblock reveal that the Zunhua–Qinglong microblock experienced three major periods of juvenile crustal growth during Neoarchean at 2.72–2.70Ga, 2.64–2.56Ga and 2.53–2.50Ga, respectively, with the latter two periods coupled with reworking of old crustal materials.

Early to Middle Paleozoic arc magmatism in the Korean Peninsula: Constraints from zircon geochronology and geochemistry

The Hongseong area of the central southwestern Korean Peninsula has been tectonically correlated to the Qinling–Dabie–Sulu collisional belt of the central China Orogenic Belt. Here we have presented geochemical and sensitive high-resolution ion microprobe (SHRIMP) U–Pb zircon data on the metaigneous rocks from the Early to Middle Paleozoic Gwangcheon gneiss, located in the middle of the Hongseong area. Our data show U–Pb ages of ca. 450Ma for quartz-dioritic gneiss, 442Ma for metagabbro, 409, 407 and 402Ma for tonalitic gneiss, 440Ma for intermediate dyke, and 401Ma for mafic intrusive. The rocks display geochemical features suggesting magma generation within Late Ordovician (ca. 450Ma) to Early Devonian (ca. 401Ma) subduction-related volcanic arc and subsequent collapsing environment. The ages and geochemical characteristics presented in this study correlate well with those of the Early to Middle Paleozoic orogenic belts in the Qinling orogenic belt. Our study provides further insights into the tectonic linkage among Paleozoic to Triassic East Asian continents, with evidence for a Paleozoic Pacific-type subduction prior to the Triassic Alpine-type collision.

The Algodões amphibolite–tonalite gneiss sequence, Borborema Province, NE Brazil: Geochemical and geochronological evidence for Palaeoproterozoic accretion of oceanic plateau/back-arc basalts and adakitic plutons

Field relationships, geochemistry and U–Pb zircon and Sm–Nd whole-rock geochronology are used to constrain the genesis of Palaeoproterozoic massive amphibolites and orthogneisses of the Algodões Sequence, Central Ceará Domain, Northeastern Brazil. The 2236 ± 55 Ma-old Algodões amphibolites show trace element geochemical signature and positive εNd( t) values similar to Phanaerozoic oceanic plateau basalts and less often to back-arc basalts. The amphibolites are intruded by tonalitic to quartz-dioritic gneisses of the Cipó unit which were dated at 2160–2170 Ma and 2130 Ma. Neodimium isotope data and whole-rock geochemistry for these gneisses show they are akin to juvenile arc plutonics of the adakitic suite. The likely occurrence of Palaeoproterozoic plateau basalts and arc plutonics in this part of northeastearn Brazil render comparisons with similar terranes elsewhere in South America and Africa, which in turn indicate a significant contribution of accretionary orogens in the early assembly of the Columbia Supercontinent.

Geochemical constraints on the petrogenesis of the Proterozoic granitoid gneisses from the eastern segment of the Central Tianshan Tectonic Zone, northwestern China

The Tianshan orogen is divided into the Northern, Central and Southern Tianshan tectonic zones by the northern and southern sutures on both sides of the Central Tianshan Tectonic Zone. The eastern segment of the Central Tianshan Tectonic Zone is characterized by the presence of numerous Precambrian metamorphic blocks and is unconformably overlain by Ordovician–Silurian and late Palaeozoic strata. The Precambrian Kumishi and Pargantag metamorphic blocks are the largest older blocks in the eastern segment of the Central Tianshan Tectonic Zone, consisting mainly of metamorphic granitoids and sedimentary rocks in greenschist to amphibolite facies. There are two major lithological assemblages of the metamorphic granitoids: (1) quartz dioritic gneisses, and (2) granodioritic–monzogranitic gneisses with a minor amount of tonalitic and syenogranitic gneisses in both the Kumishi and Pargantag blocks. The quartz dioritic gneisses are characterized by low Sr/Ce (&lt;5.3) and Sr/Y (&lt;28), relatively high Mg no. (51.0–57.0), K2O (2.65–4.04 wt %) contents and εNd(t) values (−2.37–5.84), and negative Nb and Zr–Hf anomalies, as well as relatively flat chondrite-normalized REE patterns with slightly negative Eu anomalies, suggesting that the quartz dioritic gneisses were derived from partial melting of a depleted mantle source enriched by fluids and sedimentary melts from the subducted slab. However, most of granitic gneiss samples display high K2O contents, low Al2O3/(FeO* + MgO + TiO2) values, and relatively flat chondrite-normalized REE patterns with intensively negative Eu anomalies. Integrated low εNd(t) values and older TDM model ages suggest that crustal materials played a significant role in the petrogenesis of these granitoid gneisses and that they were mainly derived from the partial melting of calc-alkaline mafic to intermediate rocks in the crust. Also, variations in geochemical features between the Kumishi–Gangou and Pargantag regions, such as Zr and Hf, may reflect geographic variability in the development of coeval granitic magmas. Tectonic discrimination for granitoid, using trace elements, together with Nd isotopic data, demonstrates that these granitoid gneisses in the eastern segment of the Central Tianshan Tectonic Zone formed in a continental margin arc during late Mesoproterozoic times.

Geology and zircon geochronology of the Acasta Gneiss Complex, northwestern Canada: New constraints on its tectonothermal history

The Acasta Gneiss Complex of northwestern Canada contains the oldest known crustal rocks on Earth. Here we present a detailed geological map of the main area of the complex (around the sample locality of the oldest known rocks) and detailed sketch maps of critical geological outcrops. The geological map shows that the complex is divided, by a northeast-trending fault, into eastern and western domains. The eastern area is comprised from quartz dioritic–gabbroic gneisses and multi-phase tonalitic–granitic gneisses. The western area is comprised of layered quartz dioritic–dioritic and tonalitic–granitic gneisses and younger foliated granitic intrusions. The detail field observations reveal at least five tectonothermal events in the eastern area: (1 and 2) emplacement of mafic-intermediate magma (protolith of the quartz dioritic gneiss) and emplacement of felsic magma (protolith of the older felsic gneiss); (3) metamorphism to produce the gneissic structures of the felsic gneiss and quartz dioritic gneiss; (4) intrusion of felsic magma (protolith of the younger felsic gneiss), causing anatexis in some parts; (5) metamorphism and deformation to produce the gneissic structure of the younger felsic gneiss. In contrast, at least four tectonothermal events have been recognized in the western area: (1 and 2) emplacement of the protolith to the mafic-intermediate and felsic gneiss; (3) metamorphism and deformation to form the gneissic and layered structures; (4) intrusion of the granite sheet (the protolith of the foliated granite); (5) metamorphism and deformation of all lithologies. To constrain the timing of the tectonothermal events, we have carried out U–Pb dating combined with cathodoluminescence imagery on zircon extracted from the gneisses and foliated granites. Our data reveal at least four tonalite–granite emplacement events in the eastern area, at ca. 3.94, 3.74–3.73, 3.66 and 3.59 Ga, and tonalite emplacement at ca. 3.97 Ga and granite intrusion at 3.58 Ga in the western area. The field relationships between the felsic and quartz dioritic gneisses in the eastern area demonstrate that the two quartz dioritic gneiss protoliths were emplaced prior to 3.59 and 3.66 Ga, respectively. These results confirm findings of previous zircon geochronology that the protolith ages of Acasta gneisses are 4.03–3.94, 3.74–3.72 and ca. 3.6 Ga. In addition, our comprehensive field observations, zircon internal structures and zircon U–Pb dating clearly demonstrate that the early Archean Acasta rocks suffered anatexis/recrystallization, coincident with the emplacement of younger felsic intrusions, and that the 3.59 Ga granitic gneiss protolith in the eastern area contains zircon xenocrysts with ages up to 3.9 Ga. In this context, we also discuss the tectonothermal evolution of the Acasta Gneiss Complex on the basis of these results and those from previous studies, and its implications for radiogenic isotopic studies.

Initial Pb of the Amı̂tsoq gneiss revisited: implications for the timing of early Archaean crustal evolution in West Greenland — Reply

Abstract The rapid change in 207 Pb / 206 Pb ratio during early Earth history provides a powerful constraint for temporal resolution of Archaean crustal evolution. New and published data for Amitsoq gneiss feldspars and whole rocks define the Earth's least radiogenic silicate Pb known until now. The most important feature of the data is that independent of the exact slope of the regression in a common Pb diagram, a very well-defined intercept with plausible terrestrial Pb evolution lines is obtained between 3.63 and 3.66 Ga, identical to a combined Pb/Pb regression age of 3654±73 Ma (MSWD=17.6). We conclude that the magmatic precursors of the Amitsoq orthogneisses separated from a depleted mantle-like source at ca. 3.65 Ga. This contrasts sharply with claims based on ion-probe U–Pb zircon dating that the Amitsoq gneisses had a prolonged, episodic, magmatic evolution extending from ca. 3.87 to 3.60 Ga. Pb isotopes offer no evidence that any Amitsoq gneiss analysed until now began its evolution in the crust as long ago as 3.85 Ga. We propose that zircons significantly older than 3.65 Ga were inherited from chemically evolved rocks as old as ca. 3.85 Ga. Geochemical arguments are presented to support this proposal. New feldspar and whole rock Pb isotope data are also reported for sheets of quartz-dioritic gneiss on Akilia Island that yield the oldest zircon U–Pb age clusters in the range 3.84–3.87 Ga. These sheets are discordant to a metasedimentary enclave containing accessory apatite with graphite inclusions whose C-isotope ratios are regarded as biogenic in origin. As above, Pb-isotopes in the sheets are consistent with a magmatic age of 3.65 Ga, which we regard as the true minimum age for these waterlain sediments and their possibly biogenic markers. Additional Pb-isotope data, as well as limited published Sm–Nd and U–Pb zircon data on the Akilia enclaves themselves, suggest an age between ca. 3.65 and 3.70 Ga for their original deposition, some 150–200 Ma younger than proposed previously. Thus the question of overlap of earliest life with a bolide impact scenario terminating ≥3.80 Ga (as on the moon) becomes irrelevant.

Initial Pb of the Amı̂tsoq gneiss revisited: implication for the timing of early Archaean crustal evolution in West Greenland

The rapid change in 207 Pb/ 206 Pb ratio during early Earth history provides a powerful constraint for temporal resolution of Archaean crustal evolution. New and published data for Amı̂tsoq gneiss feldspars and whole rocks define the Earth's least radiogenic silicate Pb known until now. The most important feature of the data is that independent of the exact slope of the regression in a common Pb diagram, a very well-defined intercept with plausible terrestrial Pb evolution lines is obtained between 3.63 and 3.66 Ga, identical to a combined Pb/Pb regression age of 3654±73 Ma (MSWD=17.6). We conclude that the magmatic precursors of the Amı̂tsoq orthogneisses separated from a depleted mantle-like source at ca. 3.65 Ga. This contrasts sharply with claims based on ion-probe U–Pb zircon dating that the Amı̂tsoq gneisses had a prolonged, episodic, magmatic evolution extending from ca. 3.87 to 3.60 Ga. Pb isotopes offer no evidence that any Amı̂tsoq gneiss analysed until now began its evolution in the crust as long ago as 3.85 Ga. We propose that zircons significantly older than 3.65 Ga were inherited from chemically evolved rocks as old as ca. 3.85 Ga. Geochemical arguments are presented to support this proposal. New feldspar and whole rock Pb isotope data are also reported for sheets of quartz-dioritic gneiss on Akilia Island that yield the oldest zircon U–Pb age clusters in the range 3.84–3.87 Ga. These sheets are discordant to a metasedimentary enclave containing accessory apatite with graphite inclusions whose C-isotope ratios are regarded as biogenic in origin. As above, Pb-isotopes in the sheets are consistent with a magmatic age of 3.65 Ga, which we regard as the true minimum age for these waterlain sediments and their possibly biogenic markers. Additional Pb-isotope data, as well as limited published Sm–Nd and U–Pb zircon data on the Akilia enclaves themselves, suggest an age between ca. 3.65 and 3.70 Ga for their original deposition, some 150–200 Ma younger than proposed previously. Thus the question of overlap of earliest life with a bolide impact scenario terminating ≥3.80 Ga (as on the moon) becomes irrelevant.

Sm-Nd isotope study of early Archean rocks, Qianan, Hebei Province, China

A Sm-Nd study of mafic amphibolite which occurs as lensoid masses in migmatitic quartz dioriticto-granitic gneiss was undertaken in order to determine if it has an early Archean age. Six amphibolite samples define an isochron yielding an age of 3.50 ± 0.08 b.y. with an initial ϵ Nd value of +3.3 ± 0.3.. Three samples of quartz dioritic gneiss lie close to this isochron, and have Sm-Nd model ages of ca. 3.5 b.y. referenced to a “epleted mantle” source. A sample of K-feldspar-rich granitic gneiss gives a younger model age of about 2.9 b.y. The non-zero initial ϵ Nd value of the mafic amphibolites can be compared with chondritic meteorite samples measured in this laboratory, and is, therefore, precisely determined. If the initial ϵ Nd value can be regarded as representative of the original igneous rocks, then it indicates that the mantle magma source was severely depleted with respect to magmaphile elements. This suggests that the geochemistry of the early Archean mantle may have been similar to that of the modern upper mantle and may require that a large pre-Archean continental crust have been present on the earth.