James K. Mortensen

UPb zircon geochronology of Archaean felsic units in the Marble Bar region, Pilbara Craton, Western Australia☆

Abstract Archaean supracrustal rocks that are well exposed in the Marble Bar region, Pilbara Craton, have been assigned to the Warrawoona Group and younger sequences. The predominantly volcanic Warrawoona Group, previously dated at 3300 to 3500 Ma, is largely basaltic with locally intercalated thick felsic volcanic units. The supracrustal rocks have been folded and are distributed around the margins of large, roughly circular to ovoid, “granitic-gneissic” batholiths. A UPb zircon geochronology study was undertaken to obtain precise age constraints for some of the ore deposits in the area, especially the Big Stubby, North Pole (barite), and Miralga Creek (ZnPbCuAu) deposits, in support of efforts to improve Archaean lead isotope models. The results also help significantly in interpreting stratigraphic relationships and crustal evolution in the area. The new conventional zircon UPb data indicate that most of the Warrawoona Group (from the Duffer Formation upwards) was deposited over a period of ∼20 Ma, from ∼3470 Ma to 3450 Ma. The age of the Duffer Formation is closely constrained by results of 3471±5 Ma and 3465±3 Ma, and the age of the upper Salgash Subgroup appears to be established at 3454±1 Ma. The Panorama Formation at the southern margin of the North Pole Dome is 3458±1.9 Ma; unless the Duffer Formation was deposited over a period of at least 10 Ma, this casts doubt on a previously suggested Panorama-Duffer correlation. The age of the lower part of the North Pole succession is greater than ∼3458 Ma, consistent with the correlation of the basal greenstones at North Pole with the Talga Talga Subgroup. The North Pole chert-barite unit, which is well known for its preservation of Early Archean stromatolites, microfossils, and evaporite sediments, is clearly older than 3458 Ma. The 3325 Ma age now established for the Wyman Formation shows that this unit must be excluded from the Warrawoona Group. The age of the Talga Talga Subgroup remains uncertain; a felsic schist in the Mount Ada Basalt, which has been dated at 3449±3 Ma, is interpreted to be a sill, probably related to the 3450 Ma granitoid bodies. An age > 3724 Ma for a zircon xenocryst in the Panorama Formation is the oldest obtained so far for zircon from the Pilbara granite-greenstone terrane, and indicates the existence of pre-Pilbara Supergroup sialic basement. Ages of 3465±3 Ma and 3449±2 Ma applied to previous Pb isotope data for the Big Stubby and Miralga Creek deposits, respectively, in combination with comparable data for other syngenetic Archaean sulphide deposits, yield a general single-stage Pb evolution model with the parameters T0=4560 Ma, A0=9.0818, B0=9.9002 and C0=29.343. This gives model ages in accord with known constraints for many Archean deposits. A model lead age of 3490 Ma for the North Pole deposit, which is older than 3457 Ma, may be too great, but attempts to directly date this deposit have not yet succeeded. There are striking parallels between the Warrawoona Group ages reported here and those recently obtained for the Barberton region, Kaapvaal Craton, South Africa. The Talga Talga Subgroup and Lower Onverwacht (Tjakastad Subgroup) sequences have approximately equivalent constraints, and an age for the upper Salgash Subgroup agrees closely with those for the Hoogenoeg Formation of the upper Onverwacht Group (Geluk Subgroup).

Pre‐Mid‐Mesozoic tectonic evolution of the Yukon‐Tanana Terrane, Yukon and Alaska

Yukon-Tanana Terrane (YTT) underlies much of central and western Yukon and east central Alaska. Its history and tectonic evolution, particularly prior to mid-Mesozoic time, has been largely obscured by younger magmatism and tectonism. The application of geochronological and isotopic techniques over the past decade, together with detailed field studies in certain critical areas of the terrane, has shed new light on the early history of YTT. Much of YTT is a product of episodic continental arc magmatism, with three main pulses in Late Devonian-Early Mississippian, mid-Permian, and Late Triassic-Early Jurassic time. From Late Devonian to mid-Mississippian time, subduction was north or northeast dipping, but arc polarity was apparently reversed by mid-Permian time. The main, subhorizontal structural fabric characterizing much of YTT was produced between mid-Permian time and the onset of renewed magmatism in Late Triassic time and probably reflects a major continent-continent collision. Although the Triassic-Jurassic magmatism is also considered to be arc related, it occurred over a very broad area of not only YTT, but also Quesnellia, and the Stikine, Nisling, Cache Creek, and Slide Mountain terranes. This magmatism appears to have coincided with final amalgamation of the Intermontane Superterrane, and the arc polarity and the position and orientation of the associated subduction zone is still controversial. Available evidence suggests that Nisling Terrane is closely related to YTT and mainly consists of older strata that underlie the Devonian and younger units generally considered to be more typical of YTT. There are close similarities between YTT and a number of other “pericratonic” terranes in the central and eastern parts of the Cordillera, and it is likely that these terranes originally formed a single arc and arc basement assemblage which has now been fragmented and dispersed by transcurrent faulting.

Timing the end-Triassic mass extinction: First on land, then in the sea?

The end-Triassic marks one of the five biggest mass extinctions, but current geologic time scales are inadequate for understanding its dynamics. A tuff layer in marine sedimentary rocks encompassing the Triassic-Jurassic transition yielded a U-Pb zircon age of 199.6 ± 0.3 Ma. The dated level is immediately below a prominent change in radiolarian faunas and the last occurrence of conodonts. Additional recently obtained U-Pb ages integrated with ammonoid biochronology confirm that the Triassic Period ended ca. 200 Ma, several million years later than suggested by previous time scales. Published dating of continental sections suggests that the extinction peak of terrestrial plants and vertebrates occurred before 200.6 Ma. The end-Triassic biotic crisis on land therefore appears to have preceded that in the sea by at least several hundred thousand years.

Minto block, Superior province: Missing link in deciphering assembly of the craton at 2.7 Ga

Plate-tectonic models of the Superior province are rooted in the granite-greenstone and metasedimentary belts of the southern part of the craton. North-striking domains of the Minto block in the northeastern part of the province evolved at similar times, in 3.1-2.8, 2.725, and 2.69 Ga events, requiring expansion of models of late Archean assembly to accommodate Minto geology. Western and eastern protocratons (∼3.1-2.8 Ga) rifted at ∼2.79 Ga to produce an ocean basin that was mostly consumed by subduction at 2.725 Ga. The Leaf River plutonic suite of cale-alkalic hornblende + biotite ± orthopyroxene ±clinopyroxene granodiorite represents magmatic arcs built on the protocratons, whereas the intervening Goudalie domain—containing fault-bounded fragments of rifted continental crust, rift volcanics, primitive oceanic crust, 2724 Ma island-arc rocks, and a <2718 Ma back-arc assemblage—marks the suture. Terminal collision at ∼2.7 Ga led to thickening and crustally derived granitoid magmatism. The southern Superior province also experienced vigorous activity between 2.725 and 2.69 Ga as island arcs, oceanic plateaus, continental fragments, and accretionary prisms were amalgamated progressively from north to south in a regime of dextral transpression then stitched by granites. A northern proto-Superior craton had continental magmatic arcs built on its eastern and southern flanks in response to west-northwest-directed subduction; orthogonal convergence in the east produced wide plutonic arcs, in contrast to terrane-accretion tectonics in the more oblique regime along the southern margin.

ARCHEAN TERRANE DOCKING : UPPER CRUST COLLISION TECTONICS, ABITIBI GREENSTONE BELT, QUEBEC, CANADA

Abstract The northern (NVZ) and southern volcanic zones (SVZ) of the Abitibi greenstone belt are separated by the major E-trending Destor-Porcupine-Manneville fault zone (DPMFZ). The DPMFZ is interpreted to be the locus of Archean terrane docking between the older diffuse volcanic arc of the NVZ (2730-2710 Ma) and the younger arc segments of the SVZ (2705-2698 Ma). Two distinct evolutionary phases can be documented along the DPMFZ of the Abitibi greenstone belt and include (1) arc-arc collision occurring between 2697 and 2690 Ma, and (2) arc fragmentation between 2689 and 2680 Ma. Identification of these two events along the DPMFZ is based on detailed structural studies, sedimentary basin analysis, and precise UPb age determinations. The thrusting event, representative of the arc-arc collision phase, is characterized by shallow north-dipping foliations (20–40°) and dip-parallel stretching lineations in the eastern Manneville segment of the DPMFZ. Local overturned mafic pillowed units suggest recumbent folding. Late strike-slip or transcurrent movement displayed in the late-orogenic sedimentary Duparquet Basin records the arc fragmentation phase. Basin geometry, E-trending en-echelon folds, shallow E-plunging stretching lineations and a late NE-striking cleavage cross-cutting the folds support a dextral shear sense along the western Destor-Porcupine segment of the DPMFZ. The sedimentary facies observed in the basin are consistent with those of modern strike-slip basins located along the East Anatolian fault, Turkey (Hazar Lake) and the Hope fault, New Zealand (Hanmer Basin). Precise UPb zircon age determinations from porphyry stocks located at the northern and southern limits of the Duparquet Basin, yielded 2681 ± 1 Ma and 2689+3.2−2.9 Ma, respectively. These ages constrain the rapid change from thrusting to transcurrent movement. It is apparent that once thrusting ceased the response to oblique subduction continued in the form of strike-slip displacement. Modern fold and thrust belts commonly show this evolution. The deformation pattern is the result of oblique convergence. The Abitibi greenstone belt is considered to be an Archean analogue of modern subducting oceanic plates such as those found in the western Pacific.

Early Mesoproterozoic intrusive breccias in Yukon, Canada: the role of hydrothermal systems in reconstructions of North America and Australia

In northern Yukon, Canada, numerous breccia zones of early Mesoproterozoic age (ca. 1.6 Ga) are targets for mineral exploration. Collectively termed Wernecke Breccia, they are characterized by disseminated specular hematite and local enrichment of Cu, Co, U and Au. The breccias are hosted mainly by the Paleoproterozoic Wernecke Supergroup, a 13-km thick basinal to platformal succession of carbonate and fine-grained clastic rocks. Brecciation occurred after the Wernecke Supergroup was fully lithified, deformed, and locally metamorphosed. The breccia zones were generated by forceful explosions of volatile-rich fluids within the crust. The source of the fluids is uncertain, but may be related to igneous intrusions at depth. Rapid expansion of the fluids shattered large volumes of country rock, mainly sedimentary rocks of the Wernecke Supergroup, and dioritic to syenitic rocks of the Bonnet Plume River intrusions. In the central parts of the breccia zones, fragments underwent considerable motion, and in some cases became rounded from abrasion. Venting of brecciated rock and fluid is considered likely, but surface deposits are nowhere preserved. At one locality, large blocks of country rock foundered into open space near the top of a breccia zone, forming a fallback megabreccia. Faulting may have been active concurrently with brecciation. Breccia fragments are cemented together by hematite, quartz, carbonate, chlorite, feldspar, mica, and other minerals. In most cases, clasts and wallrocks were hydrothermally altered, leading to metasomatic growth of secondary minerals including flecks of hematite or rhombs of dolomite. Widely disseminated earthy hematite and local potassic alteration in the breccia clasts resulted in color changes from original drab hues of gray and brown to striking pink and red. Clasts with embayments rimmed with secondary minerals such as specular hematite are evidence for the dissolution of clasts or their diagenetic cements by hydrothermal fluids. The main phase of brecciation and metasomatism occurred at ca. 1.6 Ga, as indicated by a 15955 Ma U-Pb date on titanite. Subsequent minor hydrothermal events related to emplacement of the Hart River intrusions and Bear River dykes occurred at 1382.87.4 Ma (U-Pb rutile) and ca. 1270 Ma (U-Pb baddeleyite), respectively. Mineralized breccias at and near the Olympic Dam deposit in South Australia mineralogically and texturally resemble, and have nearly the same age as, the Wernecke Breccias. These similarities suggest that both breccia provinces developed from related systems of hydrothermal activity, and provide additional evidence for models linking the cratons of North America and Australia in Proterozoic time.

Detrital zircon geochronology of the western Ellesmerian clastic wedge, northwestern Canada: Insights on Arctic tectonics and the evolution of the northern Cordilleran miogeocline

Detrital zircon provenance investigations of mid-Paleozoic sandstone from the western Ellesmerian clastic wedge and Cordilleran miogeocline in northern Yukon and Northwest Territories, northwestern Canada, provide critical new data on the source of foreland basin sedimentation attributed to terrane accretion and plate convergence along the ancestral Arctic margin of North America. Late Devonian and early Mississippian clastic wedge strata yield “exotic” ca. 360–390, 430–460, 530–680, and 1500–1600 Ma detrital zircon populations that are consistent with source rocks that originated near the Caledonian and Timanian orogenic belts. Specifically, the Pearya and Arctic Alaska–Chukotka terranes, the landmass of Crockerland, and Caledonian rocks in eastern Greenland are the inferred sources for exotic detrital zircons in clastic wedge strata. Progressive recycling of Ellesmerian foreland basin sediments into the continental margin environment along northwestern Laurentia is indicated by the presence of ca. 360–430 Ma and 1500–1600 Ma detrital zircons in post-tectonic, middle to late Mississippian miogeoclinal strata in Yukon. Provenance data from these Mississippian samples record a dramatic shift in the source of the Cordilleran miogeocline, since Caledonian and Baltican (Timanide) detrital zircon signatures are not recognized in pre–Late Devonian sedimentary rocks in western Canada. Devonian strata of the Alexander terrane and Yreka subterrane (eastern Klamath terrane) have Caledonian and Baltican detrital zircon age signatures similar to Ellesmerian clastic wedge sandstones, implying that several Cordilleran terranes originated in the paleo-Arctic realm. Speculative correlations suggest that the Arctic Alaska–Chukotka terrane was located to the west of Crockerland and the Canadian Arctic Islands in pre-Cretaceous time, prior to opening of the Amerasian basin. Rifting models for the western Arctic Ocean featuring counterclockwise rotation of the Arctic Alaska–Chukotka terrane away from the Canadian Arctic Islands may need reevaluation.

Evolution of the Yukon-Tanana terrane: Evidence from southeastern Yukon Territory

The Yukon-Tanana terrane (YTT) in southeastern Yukon consists predominantly of a mid-Paleozoic volcanic-plutonic (arc?) assemblage built on continental crust. The YTT had experienced strong deformation and metamorphism by Late Triassic time. By mid-Cretaceous the metamorphic complex and Late Triassic clastic rocks derived from it were imbricated with middle and upper Paleozoic “ophiolitic” sheets. The YTT differs markedly from adjacent parts of the North American continental margin in both its depositional and tectonic histories. The composition and tectonic history of the YTT is comparable to that of the Kootenay and Barkerville terranes of southern and central British Columbia from which the YTT may have been displaced.

Geochemical evolution of the Minto block: a 2.7 Ga continental magmatic arc built on the Superior proto-craton

Abstract A 400 km-long by 100 km-wide geological transect across the Minto block of northeastern Superior Province reveals five plutonic suites and minor amphibolite- to granulite-grade supracrustal remnants. The oldest regionally significant plutonic rocks include 3.0±0.1 Ga foliated to gneissic tonalite characterized by high Na K ratios, high Al contents, light REE enrichment, and initial geNd= +1.8. The Leaf River plutonic suite is the most widespread, with crystallization ages of about 2725 ± 5 Ma across the transect. The suite comprises massive to foliated pyroxene- and hornblende-bearing granodiorite, tonalite, granite, diorite, and minor gabbro-pyroxenite and syn-plutonic mafic dykes. The rocks are I-type, calc-alkaline, have variable Na K ratios, moderate to high Ba and Sr contents, and REE patterns with variable light REE enrichment depletion of heavy REEs with increasing silica, and neutral, negative, or positive Eu anomalies. Initial ϵNd values for the suite range from −0.5 to +1.3 (mean = +0.4), and initial 87 Sr 86 Sr ratios (ISr) are ∼0.7020. The diatexite plutonic suite (2713 Ma) occurs within a 150 km-wide zone, and consists of orthopyroxene±garnet granodiorite containing 25–50% inclusions of granulite-grade mafic, supracrustal, and tonalitic inclusions. The variable major- and trace-element chemistry of the suite, with generally high Ni and Cr contents, reflects heterogeneous source materials and the presence of non-liquid components. Initial ϵNd values are from +0.1 to −1.7. Monzogranite and orthopyroxene granite dykes, plugs and plutonic masses, both ∼2690 Ma, comprise the last important plutonic suites. Both biotite monzogranite and orthopyroxene granite are light REE-enriched, but the former have negative Eu anomalies, and the latter positive. Initial ϵNd values range from −0.7 to −3.2 for monzogranite (ISr=0.7016 to 0.7067), and +0.7 to −0.2 for orthopyroxene granite (ISr=0.701 to 0.702). The initial ϵNd and ISr values of the ∼ 2.7 Ga plutonic suites are lower and higher, respectively, than estimatesf of the contemporaneous depleted mantle, suggesting significant and wide-scale reworking of older, tight REE-enriched, high Rb SR lithosphere. Early tonalite orthogneiss from Goudalie domain may be derived by partial melting of mafic crust at high pressures. The lithological and geochemical diversity of the 2725 Ma Leaf River plutonic suite indicate a range of parental magma compositions and petrogenetic processes, but fractional crystallization of mantle-derived basaltic melts which had assimilated small amounts of significantly older lower crust may have been the dominant mechanism. Crustal-inclusion-charged granodioritic diatexite (2713 Ma) was probably derived by partial melting of heterogeneous lower crustal materials under relatively dry conditions. Late biotite monzogranite was generated by partial melting of older, sialic crust, and orthopyroxene granite by deeper melting or fractional crystallization of more juvenile source materials with garnet as a residual mineral. We infer from the presence of coeval mafic dykes in all plutonic suites that heat for melting was ultimately derived from intrusion of basaltic magmas at depth. The ∼ 2.7 Ga magmatism in the Minto block occurred during a period of active production and accretion of juvenile oceanic terranes in the southern Superior Province. The northwest-directed subduction regime inferred from the southern Superior Province arc terranes may also have been responsible for the magmatism that acted on 3 Ga lithosphere in the northern Superior Province. Melting within the mantle wedge beneath the proto-cratonic lithosphere and in the lower crust occurred within a tectonic setting similar to that of modem continental magmatic arcs.

Boninitic magmatism in a continental margin setting, Yukon- Tanana terrane, southeastern Yukon, Canada

Mid-Paleozoic mafic rocks in the Finlayson Lake region of the Yukon-Tanana terrane, southeastern Yukon, Canada, have the diagnostic geochemical signatures of boninites: high MgO, Cr, Ni, and Co contents, intermediate SiO 2 contents, high Mg#9s (MgO/ (MgO+FeO*), Al 2 O 3 /TiO 2 , and Zr(Hf)/middle rare earth element (REE) ratios; low TiO 2 , REE, and high-field-strength element contents; and U-shaped primitive mantle–normalized trace element patterns. However, unlike most modern and ancient boninitic rocks that are typically associated with intraoceanic realms, those from the Finlayson Lake region are part of a mid-Paleozoic continental margin arc-backarc magmatic system. We propose a model in which the boninitic rocks from the Finlayson Lake region formed as a result of spreading ridge propagation into an arc built on composite basement of oceanic and continental crust. In the oceanic segment, upwelling asthenosphere induced melting of a subducted-slab metasomatized refractory mantle source to form boninitic magmatism. In the continental sector, upwelling asthenospheric mantle, and/or the melts derived thereof, induced crustal melting, which explains the large volume of temporally equivalent felsic volcanic and intrusive rocks.