Evolution of a ~2.7 Ga large igneous province: A volcanological, geochemical and geochronological study of the Agnew Greenstone Belt, and new regional correlations for the Kalgoorlie Terrane (Yilgarn Craton, Western Australia)

The thick package of ~2.7 Ga mafic and ultramafic lavas and intrusions preserved among the Neoarchean of the Kalgoorlie Terrene in Western Australia provides valuable insight into geological processes controlling the most prodigious episode of growth and preservation of juvenile continental crust in Earth’s history. Limited exposure of these rocks results in uncertainty about their age, physical and chemical characteristics, and stratigraphic relationships. This in turn prevents confident correlation of regional occurrences of mafic and ultramafic successions (both intrusive and extrusive) and hinders the interpretation of tectonic setting and magmatic evolution. A recent stratigraphic drilling program of the Neoarchean stratigraphy of the Agnew Greenstone Belt in Western Australia has provided continuous exposures through a c. 7 km thick sequence of mafic and ultramafic units. In this study, we present a volcanological, lithogeochemical and chronological study of the Agnew Greenstone Belt, and provide the first pre-2690 Ma regional correlation across the Kalgoorlie Terrane. The Agnew Greenstone Belt records ~30 m.y. of episodic ultramafic-mafic magmatism that includes two cycles, each defined by a komatiite that is overlain by units that become more evolved and contaminated with time. The sequence is divided into nine conformable packages, each consisting of stacked subaqueous lava flows and comagmatic intrusions, as well as two sills without associated extrusions. Lavas, with the exception of intercalations between two units, form a layer-cake stratigraphy and were likely erupted from a system of fissures tapping the same magma source. The komatiites are not contaminated by continental crust ([La/Sm]PM ~0.7) and are of the Al-undepleted Munro-type. Crustal contamination is evident in many units (Songvang Basalt, Never Can Tell Basalt, Redeemer Basalt, and Turrett Dolerite), as judged by [La/Sm]>1, negative Nb and Ti anomalies, and geochemical mixing trends towards felsic contaminants. Crystal fractionation was also significant, with early olivine and chromite (Mg#>65) followed by plagioclase and clinopyroxene removal (Mg<65), and in the most evolved case, titanomagnetite accumulation. Three new TIMS dates on granophyric zones of mafic sills and one ICP-MS date from an interflow felsic tuff are presented and used for regional stratigraphic correlation. Cycle I magmatism began at ~2720 Ma and ended ~2705 Ma, whereas cycle II began ~2705 Ma and ended at 2690.7±1.2 Ma. Regional correlations indicate the western Kalgoorlie Terrane consists of a remarkably similar stratigraphy that can be recognised at Agnew, Ora Banda and Coolgardie, whereas the eastern part of the terrane (e.g., Kambalda Domain) does not include cycle I, but correlates well with cycle II. This research supports an autochthonous model of greenstone formation, in which one large igneous province, represented by two complete cycles, is constructed on sialic crust. New stratigraphic correlations for the Kalgoorlie Terrane indicate that many units can be traced over distances >100 km, which has implications for exploration targeting for stratigraphically hosted ultramafic Ni and VMS deposits.

Two cycles of voluminous pyroclastic volcanism and sedimentation related to episodic granite emplacement during the late Archean : Eastern Yilgarn Craton, Western Australia

The thick piles of late-Archean volcaniclastic sedimentary successions that overlie the voluminous greenstone units of the eastern Yilgarn Craton, Western Australia, record the important transition from the cessation in mafic-ultramafic volcanism to cratonisation between about 2690 and 2655 Ma. Unfortunately, an inability to clearly subdivide the superficially similar sedimentary successions and correlate them between the various geological terranes and domains of the eastern Yilgarn Craton has led to uncertainty about the timing and nature of the region's palaeogeographic and palaeotectonic evolution. Here, we present the results of some 2025 U–Pb laser-ablation-ICP-MS analyses and 323 Sensitive High-Resolution Ion Microprobe (SHRIMP) analyses of detrital zircons from 14 late-Archean felsic clastic successions of the eastern Yilgarn Craton, which have enabled correlation of clastic successions. The results of our data, together with those compiled from previous studies, show that the post-greenstone sedimentary successions include two major cycles that both commenced with voluminous pyroclastic volcanism and ended with widespread exhumation and erosion associated with granite emplacement. Cycle One commences with an influx of rapidly reworked feldspar-rich pyroclastic debris. These units, here-named the Early Black Flag Group, are dominated by a single population of detrital zircons with an average age of 2690–2680 Ma. Thick (up to 2 km) dolerite bodies, such as the Golden Mile Dolerite, intrude the upper parts of the Early Black Flag Group at about 2680 Ma. Incipient development of large granite domes during Cycle One created extensional basins predominantly near their southeastern and northwestern margins (e.g., St Ives, Wallaby, Kanowna Belle and Agnew), into which the Early Black Flag Group and overlying coarse mafic conglomerate facies of the Late Black Flag Group were deposited. The clast compositions and detrital-zircon ages of the late Black Flag Group detritus match closely the nearby and/or stratigraphically underlying successions, thus suggesting relatively local provenance. Cycle Two involved a similar progression to that observed in Cycle One, but the age and composition of the detritus were notably different. Deposition of rapidly reworked quartz-rich pyroclastic deposits dominated by a single detrital-zircon age population of 2670–2660 Ma heralded the beginning of Cycle Two. These coarse-grained quartz-rich units, are name here the Early Merougil Group. The mean ages of the detrital zircons from the Early Merougil Group match closely the age of the peak in high-Ca (quartz-rich) granite magmatism in the Yilgarn Craton and thus probably represent the surface expression of the same event. Successions of the Late Merougil Group are dominated by coarse felsic conglomerate with abundant volcanic quartz. Although the detrital zircons in these successions have a broad spread of age, the principal sub-populations have ages of about 2665 Ma and thus match closely those of the Early Merougil Group. These successions occur most commonly at the northwestern and southeastern margins of the granite batholiths and thus are interpreted to represent resedimented units dominted by the stratigraphically underlying packages of the Early Merougil Group. The Kurrawang Group is the youngest sedimentary units identified in this study and is dominated by polymictic conglomerate with clasts of banded iron formation (BIF), granite and quartzite near the base and quartz-rich sandstone units containing detrital zircons aged up to 3500 Ma near the top. These units record provenance from deeper and/or more-distal sources. We suggest here that the principal driver for the major episodes of volcanism, sedimentation and deformation associated with basin development was the progressive emplacement of large granite batholiths. This interpretation has important implication for palaeogeographic and palaeotectonic evolution of all late-Archean terranes around the world.

Distinguishing between mechanisms of cell aggregation using pair-correlation functions

Many cell types form clumps or aggregates when cultured in vitro through a variety of mechanisms including rapid cell proliferation, chemotaxis, or direct cell-to-cell contact. In this paper we develop an agent-based model to explore the formation of aggregates in cultures where cells are initially distributed uniformly, at random, on a two-dimensional substrate. Our model includes unbiased random cell motion, together with two mechanisms which can produce cell aggregates: (i) rapid cell proliferation, and (ii) a biased cell motility mechanism where cells can sense other cells within a finite range, and will tend to move towards areas with higher numbers of cells. We then introduce a pair-correlation function which allows us to quantify aspects of the spatial patterns produced by our agent-based model. In particular, these pair-correlation functions are able to detect differences between domains populated uniformly at random (i.e. at the exclusion complete spatial randomness (ECSR) state) and those where the proliferation and biased motion rules have been employed - even when such differences are not obvious to the naked eye. The pair-correlation function can also detect the emergence of a characteristic inter-aggregate distance which occurs when the biased motion mechanism is dominant, and is not observed when cell proliferation is the main mechanism of aggregate formation. This suggests that applying the pair-correlation function to experimental images of cell aggregates may provide information about the mechanism associated with observed aggregates. As a proof of concept, we perform such analysis for images of cancer cell aggregates, which are known to be associated with rapid proliferation. The results of our analysis are consistent with the predictions of the proliferation-based simulations, which supports the potential usefulness of pair correlation functions for providing insight into the mechanisms of aggregate formation.