Neoarchean Supracrustal Rocks Research Articles

Pressure–temperature evolution, protolith characteristics and tectonic setting of granulite facies rocks from Laojinchang area in the Longgang Block, China

The Archean supracrustal rock series in the Laojinchang area of Jilin Province, China provide valuable insights into the Precambrian geological evolution of the North China Craton. This study presents new results on petrography, geochemistry, mineralogy, isotope geochronology and phase equilibrium modeling of the Laojinchang granulite facies metamorphic rocks. The protolith ages of garnet two-pyroxene granulite and amphibolite from Sidaolazihe Formation, and two-pyroxene granulite from Laoniugou Formation are 2565 ± 5.9 Ma, 2512 ± 17 Ma, and 2509.2 ± 8.6 Ma, respectively, with the metamorphic age recorded by amphibolite being 2475.6 ± 9 Ma. New geochemical data indicates that TTGs (tonalite-trondhjemite-granodiorite) weathering in an island arc environment formed the protolith of argillaceous gneiss of Sidaolazihe Formation. Basaltic andesite, formed under an island arc environment, was the protolith of garnet two-pyroxene granulite of Sidaolazihe Formation; while calc-alkaline basalt was the protolith of amphibolite from the same formation and two-pyroxene granulite from Laoniugou Formation. All these Archean supracrustal rocks exposed in the Laojinchang area have recorded nearly isobaric cooling (IBC) anticlockwise P-T paths with distinct prograde, peak and post-peak stages. The peak P-T conditions reached granulite facies, suggesting a heat source from underplated magmas. A tectonic model involving an island-arc setting is proposed for the evolution of these Neoarchean supracrustal rocks in Laojinchang area.

Two contrasting Neoarchean metavolcanic rock suites in eastern Hebei and their geodynamic implications for the northern North China Craton

The late Archean was a crucial transition period for global geodynamics. Metavolcanic rocks, as one of the predominant components of Archean cratons, are key to deciphering the geodynamic regimes of early Earth crust-mantle interactions. Neoarchean supracrustal rocks in the central K-rich granitoid belt of the eastern Hebei-western Liaoning Province, northern North China Craton, may be subdivided into a lower metavolcanic sequence and an upper metavolcanic-sedimentary sequence by an angular unconformity.The lower sequence metavolcanic rocks consist primarily of 2558–2536 Ma amphibolite to granulite facies metamorphosed basaltic-andesitic rocks and high- and low-MgO dacitic rocks. The basaltic-andesitic samples display calc-alkaline affinity, high MgO contents, low (Nb/La)N and moderate (Hf/Sm)N values and were most likely derived from partial melting of mantle materials metasomatized by subducted slab- and sediment-derived melts and fluids. The high-MgO dacitic samples are characterized by strongly fractionated rare earth element (REE) patterns and relatively high MgO contents, which resemble typical high-SiO2 adakites. The magmatic precursors originated from subducted oceanic slabs and sediments. The samples with low-MgO dacite compositions have the highest SiO2 and lowest MgO contents, and the magmatic precursors were generated by partial melting of metamorphosed high-K mafic rocks and sedimentary rocks in the lower crustal level.The upper sequence metavolcanic rocks chiefly comprise greenschist to amphibolite facies metamorphosed tholeiites, tholeiitic to transitional basalts, calc-alkaline basalts, andesites, and dacites with magmatic crystallization ages of 2522–2511 Ma. The tholeiite samples are characterized by left-declined REE patterns and high (Nb/La)N and chondrite-like (Hf/Sm)N values resembling typical N-MORBs and were formed by partial melting of a depleted mantle source. The tholeiitic to transitional basalt samples resemble back-arc basin basalts and display weakly right-declined REE patterns, mildly negative Nb and Ti anomalies, and low (Nb/La)N and chondrite-like (Hf/Sm)N values. Their magmatic precursors were derived from partial melting of a slightly fluid- and melt-metasomatized depleted mantle source. The calc-alkaline basalt and andesite samples exhibit fractionated REE patterns and apparent Nb, Ta, and Ti depletions and originated from a strongly fluid- and melt-metasomatized depleted mantle source. The dacite samples show the highest SiO2 and lowest MgO contents in the upper sequence metavolcanic rocks and were derived from partial melting of lower crustal metasedimentary rocks. Systematic petrological, geochemical, and petrogenetic studies indicate that a mature continental arc setting and a back-arc basin setting may account for the formation of the lower and upper sequence metavolcanic rocks, respectively. Three stages of deformation can be identified in the K-rich granitoid belt, including 2525–2505 Ma extensional deformation in a back-arc basin setting (D1), ca. 2500 Ma top-to-the-NEE-SSE thrusting in a collisional orogenic setting (D2) and 2500–1800 Ma nearly N-S-oriented strike-slip shearing (D3), with D2 deformation accompanied by the synchronous occurrence of paired high-pressure and high-temperature medium-pressure granulite facies metamorphism. Integrating the petrogenesis, spatiotemporal relationships, deformational and metamorphic features of the Archean lithological assemblages, we consider that the late Neoarchean K-rich granitoid belt was formed in an active continental margin setting with a southeastward subduction polarity and experienced multi-stage trench retreats and a subsequent collision between the arc and continent.

Lithospheric modification at the onset of the destruction of the North China Craton: Evidence from Late Triassic mafic dykes

Mantle-derived magmatism provides important insights for understanding the mechanism of lithospheric thinning. Here we report the results of an integrated geochronological and geochemical study of Late Triassic mafic dykes in Eastern Hebei, northern North China Craton. In situ zircon U-Pb dating shows that the dykes were emplaced between 238 and 223 Ma; the coeval Gaojiadian and Mataizi dykes intruded Precambrian basement at 238–234 Ma and the Saheqiao dyke was emplaced into Neoarchean supracrustal rocks later at 223 ± 4 Ma (2s). Bulk-rock geochemistry indicates that the Late Triassic dykes in Eastern Hebei were produced by melting of ancient lithospheric mantle within the garnet-spinel transition zone (~70–80 km), heated by upwelling asthenosphere. This ancient lithospheric mantle had been metasomatized during previous subduction events. The Gaojiadian and Mataizi dykes resulted from higher degrees of partial melting at slightly lower pressures than the Saheqiao dyke. The melting depth of Late Triassic dykes in Eastern Hebei indicates that the intact ancient lithospheric mantle had been at least locally modified/thinned to ~70–80 km by the Late Triassic. The intrusion of these Late Triassic dykes took place at the onset of the lithospheric thinning of the North China Craton, caused by post-collisional extension after subduction and collision of neighboring blocks with the North China Craton.

Early Neoarchean oceanic crust in the North China Craton: Evidence from geology, geochemistry and geochronology of greenstone belts in western Shandong

Western Shandong is the only area in the North China Craton where abundant supracrustal rocks of the early and late Neoarchean are developed. This study presents an integrated geological investigations, whole-rock geochemistry and SHRIMP UPb zircon dating investigation of the supracrustal rocks at Yanlingguang, one of the most typical areas of Neoarchean supracrustal rocks in western Shandong. The original Yanlingguan ‘formation’ is now subdivided into three belts: the Northeastern, Central and Southwestern belts, which are all in tectonic contact. The Northeastern and Southwestern belts are mainly composed of meta-komatiitic and basaltic rocks. Four granitic dykes intruding these belts have magmatic zircon ages of 2731–2530 Ma, supporting the conclusion that the supracrustal rocks were formed in the early Neoarchean. Meta-basalt with pillow structures has been identified in several locations, with the largest being ~35 m in thickness. The meta-komatiitic rocks are Al-undepleted komatiites and have ∑REE and (La/Yb)n of 2.30–26.21 ppm and 0.29–2.35, respectively, whereas the meta-basaltic rocks have ∑REE and (La/Yb)n of 16.45–53.26 ppm and 0.67–1.78, respectively, and εNd(t) values of 3.36–4.76, being similar to N-MORB. Eleven samples of meta-sedimentary rocks (including meta-sandstones-siltstones and meta-conglomerates) from the Central Belt have zircon UPb ages extending from 2760 to 2520 Ma, providing evidence for the first time that they were deposited during the late Neoarchean, rather than the early Neoarchean as previously thought. The meta-sandstones-siltstones show large variations in major element contents, but have low ∑REE (49.17–102.30 ppm) and (La/Yb)n (1.53–7.00) ratios, with negligible to weakly negative or positive Eu anomalies (Eu/Eu* = 0.60–1.17), and record εNd(t) values of 0.96–3.21. Combined with regional geological investigations, we conclude that the early Neoarchean supracrustal rocks were formed in an oceanic environment. It is also suggested that the sandstones-siltstones of the late Neoarchean were derived from the early Neoarchean basement, and that the conglomerates were derived from two separate source regions; one of early, and the other of late Neoarchean age. The identification of basal conglomerates is consistent with the arc model established in early studies. We suggest that the criteria employed in this study can be used for determining whether or not an Archean greenstone belt is of oceanic affinity.

A back-arc origin for the Neoarchean megacrystic anorthosite-bearing Bird River Sill and the associated greenstone belt, Bird River subprovince, Western Superior Province, Manitoba, Canada

This study presents extensive new field, petrographic, and whole-rock major and trace element and Sm–Nd and Pb isotope data for the Neoarchean megacrystic anorthosite-bearing Bird River Sill, spatially associated Neoarchean supracrustal rocks of the Bird River greenstone belt, and the Mesoarchean Maskwa Lake TTG Batholith I, Manitoba, Canada. Field observations indicate that the 2743 Ma Bird River Sill was emplaced into the Northern Lamprey Falls Formation in an oceanic setting and subsequently intruded by the ca. 2725 Ma Maskwa Lake Batholith II. The Northern Lamprey Falls Formation is in fault contact with the overlying Peterson Creek Formation, which in turn is unconformably overlain by the Bird River Formation. The peridotites of the Bird River Sill were subjected to intense alteration and ductile shearing, resulting in the redistribution of many major and trace elements and resetting of their Sm–Nd and U–Th–Pb isotopic systems. The gabbro, leucogabbro, and anorthosite units of the sill largely retain their primary igneous textures and minerals, REE and HFSE compositions, and Sm–Nd and Pb isotope signatures. These geochemical and petrographic data indicate that these units of the Bird River Sill crystallised from a hydrous, Ca- and Al-rich tholeiitic magma that fractionated from a hydrous primitive arc tholeiitic parental magma. The major and trace element geochemistry and initial eNd values of the Bird River Sill (+ 0.53 to + 4.72) and the Northern Lamprey Falls Formation (+ 1.40 to + 2.66) of the Bird River greenstone belt crystallised from parental magma batches that were derived by partial melting of variably-depleted mantle sources. High-magnesian andesites from the Bird River Formation formed by partial melting of subducting oceanic crust and/or slab sediments and subsequent interaction with sub-arc mantle peridotite, whereas the Peterson Creek Formation dacites and Maskwa Lake TTG Batholith I tonalites formed by partial melting of juvenile lower arc crust. The major and trace element geochemistry of the Bird River Sill and greenstone belt and the Maskwa Lake TTG Batholith I, and geological relationships and lithological associations point to a transition from continental arc to continental back-arc magmatism along the southern active margin of the North Caribou terrane at ca. 2743 Ma, resulting in the formation of the Maskwa Lake microcontinent that occurred prior to subsequent continental arc magmatism along the southern margin of this microcontinent. Combined field observations and geochemical data indicate that the Bird River Sill and Bird River greenstone belt are a dismembered Archean subduction-related ophiolite that marks a suture zone between the Winnipeg River subprovince and the Maskwa Lake microcontinent.

A Neoarchean K-rich granitoid belt in the northern North China Craton

The Archean metamorphic basement in the southeastern part of Eastern Hebei to Western Liaoning provinces, northern North China Craton, is dominated by Neoarchean K-rich granitoids, with a small amount of Neoarchean supracrustal rocks and Na-rich dioritic-tonalitic-trondhjemitic-granodioritic gneisses, and minor Paleo- to Mesoarchean rocks. The Paleo- to Mesoarchean lithological signatures include TTG gneisses and supracrustal rocks at the Caozhuang and Caochang areas. The Neoarchean K-rich granitoids form a ca. 350 km long belt extending in a NE direction, and primarily consist of porphyritic monzodioritic-quartz monzodioritic-granodioritic-monzogranitic (MQGM) gneisses and secondary monzogranite-syenogranites and charnockites, with minor Neoarchean TTG gneiss enclaves. Based on their major mafic mineral phases, the MQGM gneisses can be divided into an amphibole-dominated group and a biotite-dominated group. Zircon U-Pb isotopic dating reveals that the Caozhuang trondhjemitic gneisses, amphibole-dominated MQGM gneisses, and biotite-dominated MQGM gneisses were emplaced at ∼2.90 Ga, 2.55–2.53 Ga, and 2.55–2.53 Ga, respectively. The amphibole-dominated porphyritic MQGM gneisses are geochemically analogous to sanukitoids, and the sanukitoid magma was derived from the partial melting of lithospheric mantle metasomatized by dehydration fluids and melts from subducted slabs and related sediments, following the fractionation of amphibole and clinopyroxene during their magmatic evolution. The magmatic precursors of the biotite-dominated MQGM gneisses were most likely generated by the partial melting of medium- to high-K mafic-intermediate rocks and locally generated by the partial melting of metagreywackes at medium pressures. Within the K-rich granitoid belt, the 2.55–2.53 Ga porphyritic MQGM gneisses experienced 2.53–2.51 Ga E-W extension and strike-slip shearing deformation in the Jinzhou-Xingcheng structural domain, and diapiric upwelling by the formation of dome structures in the Jielingkou-Anziling-Suizhong structural domain. All these structures then suffered a top-to-the-SE-NEE thrusting in the northern margin during 2.51–2.50 Ga. The 2.53–2.51 Ga dioritic-tonalitic-trondhjemitic-granodioritic gneisses and charnockites in the Taipingzhai-Santunying-Zunhua structural domain firstly underwent diapiric upwelling during 2.51–2.50 Ga, then together with the 2.52–2.50 Ga volcanic-sedimentary rocks in the Shuangshanzi-Qian’an-Lulong structural domain overprinted by nearly N-S-striking dextral strike-slip shearing deformation during 2.50–1.80 Ga. The temporal-spatial relationships, petrogenesis, and structural patterns of these various Neoarchean lithological assemblages confirm that the Neoarchean K-rich granitoid belt in the northern North China Craton was most likely developed in an active continental margin setting, and our studies suggest this involved progressively southeastward multi-stage retreating oceanic slab subduction and collisions between an oceanic arc and an ancient continental nucleus.

Geochemistry of meta-sedimentary rocks associated with the Neoarchean Dagushan BIF in the Anshan-Benxi area, North China Craton: Implications for their provenance and tectonic setting

The Anshan-Benxi area is situated in the northeastern part of the North China Craton (NCC) and consists predominantly of Neoarchean supracrustal rocks and plutons with minor Eoarchean to Mesoarchean rocks. These supracrustal rocks were originally considered to be formed synchronously (∼2.5 Ga) at different localities on an active continental margin. Recently, more detailed petrographic, geochemical, and geochronological analyses showed that the supracrustal strata in the Benxi area (2.56–2.53 Ga) were slightly older than those in the Anshan area (2.53–2.51 Ga), which further indicates that they were formed during opening and closure of the back-arc basin, respectively. However, the Dagushan banded iron formation (BIF) and associated supracrustal rocks in the Anshan area were interpreted to be deposited in Mesoarchean (∼3.10 Ga), which constructs a clear contrast with other supracrustal rocks in the Anshan-Benxi area and renders previous tectonic models controversial.On the basis of detailed field investigations in the Dagushan area, we present in situ zircon U-Pb-Hf isotopic, whole-rock geochemical and Sm-Nd isotopic data of a suite of meta-sedimentary rocks (i.e., meta-arenites, meta-shales, and high-Fe meta-shales), in order to provide constraints on their depositional age, provenance, and tectonic setting. The weighted mean U-Pb ages of the youngest group of detrital zircons for the meta-arenite in the Dagushan area, coupled with the maximum metamorphic age of ∼2.51 Ga in the Anshan-Benxi area, suggest that the Dagushan BIF was deposited between 2.53 and 2.51 Ga. Geochemical features such as the Al2O3/TiO2 and Th/Zr values, and REE patterns suggest that the meta-arenites and meta-shales were mainly derived from felsic igneous sources, whereas the high-Fe meta-shales were predominantly sourced from mafic rocks. The U-Pb age spectrum diagram of detrital zircons within meta-arenites further indicates that the dominant source rocks may be the coeval volcanic rocks in the Benxi area. Our zircon Hf isotopes, coupled with other previously published zircon Hf isotopic data from Neoarchean supracrustal rocks in the Anshan-Benxi area, suggest that major juvenile crustal growth and reworking events occurred at 2.9–2.7 Ga and ∼2.5 Ga, respectively. In all, the Dagushan meta-sedimentary rocks were probably deposited in a back-arc basin on the ancient continental margin.

Episodic crustal growth and reworking of the Yudongzi terrane, South China: Constraints from the Archean TTGs and potassic granites and Paleoproterozoic amphibolites

The Yudongzi terrane is featured by well-preserved lithologic outcrops of basement rocks, including the ~2.7 Ga tonalite-trondhjemite-granodiorite (TTGs), ~2.5 Ga potassic granites, Neoarchean supracrustal rocks and ~1.85 Ga amphibolites. The TTG rocks contain ~2.8 Ga inherited/xenocrystic zircons, which indicates the existence of ancient rocks in the magma source. Most zircon grains in the TTG rocks have chondrite-like Hf isotopic compositions, indicative of derivation of magmas from the Paleo-Mesoarchean (3.3–3.0 Ga) crustal sources, consistent with their whole-rock Nd model ages (3.3–2.9 Ga). Zircon oxygen isotopic compositions (5.4 to 6.1‰) are mainly in equilibrium with the mantle, indicating insignificant surficial processes for the source rocks. Thus, the Archean TTGs likely originated from melting of an overthickened oceanic island arc where shallow subduction occurred. The compositional diversity of the TTG rocks can be ascribed to varying residual minerals after melting of mafic precursors at different crustal depths. The subsequent intracrustal melting of the TTG-dominated proto-crusts formed more potassic granites at ~2.5 Ga, probably associated with dense residue delamination and concomitant mantle upwelling. Detrital zircon age pattern and geochemical features of the meta-sediments from the Yudongzi Group demonstrate that the adjacent meta-volcanic components are the major sedimentary source, and thus they were proximal erosion and deposition products of an arc system within a proto-subduction zone. Protolith Hf isotope signature in metamorphic zircons of the amphibolites provides evidence for the existence of early Archean basaltic proto-arc crusts which were metamorphosed at ~1.85 Ga. Collectively, the above-stated Neoarchean to Paleoproterozoic tectonothermal events imply significant crustal growth and reworking, which were coincident with global-scale continental lithospheric development and stabilization during assembly and dispersal of supercontinents Kenorland and Columbia. Similar tectonostratigraphic, paleomagnetic and geochronologic records support a Neoarchean-Paleoproterozoic connection of the Yangtze Block with the circum-N Laurentia and southern Siberia. Furthermore, the late Archean geochemical transition from sodic TTG to potassic crust record, as a sign of cratonic maturation, is collectively related to the kinematic adjustment of supercontinent process and thermal evolution of the Earth system, therefore sheding lights on subduction tectonic operation and varied crustal growth mechanism.

Late Neoarchean supracrustal rocks from the Anshan-Benxi terrane, North China Craton: New geodynamic implications from the geochemical record

The late Neoarchean supracrustal rocks of the Anshan-Benxi (AnBen) terrane in the northeastern part of the North China Craton (NCC) have important regional tectonostratigraphic implications for the development of large-scale banded iron formations (BIFs). These supracrustal rocks underwent polyphase deformation and upper greenschist to upper amphibolite facies metamorphism and are subdivided into two main lithotectonic units in terms of differing ages, metamorphic grades, and lithological assemblages: the Benxi unit (∼2.56–2.53 Ga) composed of tholeiitic to transitional mafic rocks with minor siliciclastic sedimentary rocks, and the Anshan unit (∼2.53–2.51 Ga) consisting of siliciclastic sedimentary rocks with minor tholeiitic mafic rocks. In this study, we report the results of field investigations and petrographic, whole-rock major and trace element geochemical and Sm-Nd isotopic analyses, as well as zircon U-Pb-Hf-O isotopic analyses for these volcano-sedimentary rocks, and integrate the results with previously published data to provide a new geodynamic synthesis for the setting of the BIFs. Three groups of amphibolites (meta-basalts) in the vicinity of BIFs were recognized in the AnBen area on the basis of their REE and HFSE characteristics: (a) Group 1 is characterized by nearly-flat to slightly depleted LREE patterns and an absence of significant Nb anomalies, similar to those of modern N-MORB, suggesting that their magmatic precursors were derived from partial melting of a strongly depleted mantle; (b) Group 2 displays relatively flat to slightly enriched LREE patterns and minor negative Nb anomalies, which are comparable to those of back-arc basin basalts and indicate that they were probably generated by partial melting of depleted mantle modified by slab-derived fluids; and (c) Group 3 exhibits LREE-enriched patterns and pronounced negative Nb anomalies, which are interpreted to be the result of a depleted mantle-derived melt interacting with older upper continental crust. The overall field and geochemical signatures suggest that these basaltic rocks were erupted in a back-arc basin within a continental volcanic arc setting with a protracted history, consistent with Nd isotope, inherited zircon age, and zircon Hf-O isotope data that require variable inputs of older crustal sources for their formation. Diagnostic geochemical features and detrital zircon age spectra further indicate that the source rocks for felsic to mafic detritus in the Anshan unit may have been derived dominantly from coeval volcanic rocks of the Benxi unit and/or distal volcanic arc edifices. Collectively, the evidence indicates that the tectonic evolution of the AnBen terrane took place in two main stages: the first stage involved the generation of ∼2560 to 2530 Ma tholeiitic to transitional mafic rocks and associated siliciclastic sedimentary rocks of the Benxi unit, which is interpreted to have formed during opening of the back-arc basin, whereas the second stage corresponded to closure of the back-arc basin (∼2530–2510 Ma) and involved extensive chemical and siliciclastic sedimentary rocks with minor emplacement of tholeiitic mafic lavas.

The genesis of Archean supracrustal rocks in the western Shandong Province of North China Craton: Constraints on regional crustal evolution

Archean greenstone belts are supracrustal sequences, the lower part of which is usually composed of voluminous ultramafic-mafic volcanics. Intermediate and acid volcanic rocks increase in abundance towards the upper domains. Greenstone belts constitute ~30% of the total volume of Archean cratons, and preserve significant information on the surface environment and magmatism in the early earth, which are useful in unraveling the nature of crustal formation and evolution. The western Shandong Province (WSP) is located at the eastern part of the North China Craton (NCC), where greenstone sequences formed at ~2.7 and ~2.5 Ga were well preserved. The early Neoarchean supracrustal rocks include komatiite-basalt sequence, some meta-sediments of the lower part of the Taishan Group and the Mengjiatun Formation. The volcanism had been correlated to mantle plume, which resulted in vertical crustal accretion. The late Neoarchean supracrustal rocks were composed of metamorphosed felsic volcano-sedimentary sequences and BIFs of the upper part of the Taishan Group and the Jining Group. The geochemical features of the meta-volcanics show calc-alkaline affinities, similar to modern arc-related magmatism, suggesting that the continental crust in the western Shandong Province witnessed horizontal plate movements at ~2.5 Ga. The metasediments and leucosomes in the Qixingtai area display regional upper amphibolite facies metamorphism and anatexis at 2.53–2.50 Ga, coeval with formation of large volumes of crustally-derived granites. These tectono-thermal events suggest that a unified continental crust was formed in the western Shandong Province at the end of Neoarchean.

Redefinition of depositional ages of Neoarchean supracrustal rocks in western Shandong Province, China: SHRIMP U–Pb zircon dating

Western Shandong Province is an important terrane in the eastern part of the North China Craton where 2.75–2.5Ga tectono-thermal events are well recognized. Supracrustal rocks, named the Taishan “Group”, experienced greenschist- to amphibolite-facies metamorphism and occur as lenses and strips in granitoids and are subdivided into the Yanlingguan, Shancaoyu and Liuhang “Formations”. The Yanlingguan “Formation” is mainly composed of amphibolite but also contains fine-grained biotite-hornblende gneiss, komatiitic rocks and schist. The Liuhang “Formation” is similar in rock association to the Yanlingguan “Formation” but contains more metasedimentary rocks. The Shancaoyu “Formation” is mainly composed of fine-grained biotite gneiss (leptynite) preserving bedding and is interlayered with schist. Its protoliths are mainly sedimentary rocks with some mafic to felsic volcano-sedimentary varieties. We undertook SHRIMP zircon dating on eighteen metasedimentary and volcanic samples, mainly from the Shancaoyu “Formation” and the upper part of the Liuhang “Formation”. Ages of detrital and volcanic zircons vary from 2.75 to 2.54Ga and 2.55 to 2.525Ga, respectively, and the age distribution patterns vary in different samples. We conclude that the Shancaoyu “Formation” and the upper part of the Liuhang “Formation” were deposited in the late Neoarchean (2.55–2.525Ga) and not in the early Neoarchean (2.8–2.7Ga) as thought before. Western Shandong is the only terrane in the North China Craton where both early and late Neoarchean supracrustal rocks have been identified. A magmatic arc model may be applied to explain the tectono-thermal evolution at the end of the Neoarchean.

RELATIVE AND ABSOLUTE TIMING OF GOLD MINERALIZATION ALONG THE MELIADINETREND, NUNAVUT, CANADA: EVIDENCE FOR PALEOPROTEROZOIC GOLD HOSTED IN AN ARCHEANGREENSTONE BELT

The Meliadine area of the Neoarchean Rankin Inlet greenstone belt is located on the western shore of Hudson Bay, Canada, and is host to several important orogenic or mesothermal gold deposits. Supracrustal rocks comprise alternating southeast-striking panels of mafic volcanic and clastic sedimentary rocks, both of which are host to banded iron formation. The dominant structural element is the Pyke Break, a several-kilometer-wide, high-strain zone characterized by multiple foliations and regionally important shear zones. Gold mineralization is spatially associated with these shear zones. The maximum age of mineralization is constrained by the observation that auriferous veins crosscut ca. 2450 Ma mafic dikes, and a minimum age is constrained by undeformed, postore lamprophyre dikes emplaced at ca. 1830 Ma. Highly concordant SHRIMP-determined ages on hydrothermal monazite indicate auriferous veining occurred at ca. 1850 Ma. These relative and absolute timing constraints suggest gold was concentrated in Neoarchean supracrustal rocks during Paleoproterozoic tectonothermal activity that was widespread in the western Churchill province at ca. 1850 Ma. These observations do not preclude the presence of Archean gold occurrences in the Rankin Inlet greenstone belt, but no early deformed ores can be linked to auriferous veins introduced at ca. 1850 Ma, suggesting remobilization from earlier formed deposits was not a factor in concentrating gold at Meliadine.

Dual sources of ensimatic magmas, Hearne domain, Western Churchill Province, Nunavut, Canada: Neorchean “infant arc” processes?

In the Henik segment of the Central Hearne supracrustal belt, Neoarchean supracrustal rocks accumulated in deep-water depositional systems that were well removed from a continental influence. Repeated in time and space, these systems consisted of mafic lava plain, slope, and apron subenvironments, with local emergent to near-emergent felsic volcanic centres. Volcanic and plutonic rocks are subalkaline, and range in composition from basalt to rhyolite. Consistent with field relationships, the volcanic rocks are juvenile and, with predominantly positive values that are independent of compositional variation, display little evidence of contamination by continental crust. However, despite primary interlayering on scales of 10s of metres, mafic and felsic rocks display markedly different rare earth element (REE) characteristics. The mafic volcanic rocks are MORB-like and display a tholeiitic magma evolution trend, flat REE patterns, and positive values, suggesting that they were derived from partial melting of depleted mantle. In marked contrast, the felsic rocks display steeply sloping REE patterns with significantly lower heavy REE abundances, negative Nb anomalies, and positive Zr and Hf anomalies suggesting derivation from partial melting of slightly older basaltic crust. This duality of magma sources supports a generalized model for the Central Hearne supracrustal belt that draws analogies from Eocene intraoceanic “infant arc” processes described from the southwestern Pacific Ocean, rather than from those normally associated with fully developed Phanerozoic subduction.