Microbes Caused the RIse of the First MicroContinents?

Microbes, Mineral Evolution, and the Rise of Microcontinents—Origin and Coevolution of Life with Early Earth

Authors:

Grosch et al

Abstract:

Earth is the most mineralogically diverse planet in our solar system, the direct consequence of a coevolving geosphere and biosphere. We consider the possibility that a microbial biosphere originated and thrived in the early Hadean-Archean Earth subseafloor environment, with fundamental consequences for the complex evolution and habitability of our planet. In this hypothesis paper, we explore possible venues for the origin of life and the direct consequences of microbially mediated, low-temperature hydrothermal alteration of the early oceanic lithosphere. We hypothesize that subsurface fluid-rock-microbe interactions resulted in more efficient hydration of the early oceanic crust, which in turn promoted bulk melting to produce the first evolved fragments of felsic crust. These evolved magmas most likely included sialic or tonalitic sheets, felsic volcaniclastics, and minor rhyolitic intrusions emplaced in an Iceland-type extensional setting as the earliest microcontinents. With the further development of proto-tectonic processes, these buoyant felsic crustal fragments formed the nucleus of intra-oceanic tonalite-trondhjemite-granitoid (TTG) island arcs. Thus microbes, by facilitating extensive hydrothermal alteration of the earliest oceanic crust through bioalteration, promoted mineral diversification and may have been early architects of surface environments and microcontinents on young Earth. We explore how the possible onset of subseafloor fluid-rock-microbe interactions on early Earth accelerated metavolcanic clay mineral formation, crustal melting, and subsequent metamorphic mineral evolution. We also consider environmental factors supporting this earliest step in geosphere-biosphere coevolution and the implications for habitability and mineral evolution on other rocky planets, such as Mars.

Gulden Draak Knoll Microcontinent Found Off Western Australia

Discovery of a microcontinent (Gulden Draak Knoll) offshore Western Australia: Implications for East Gondwana reconstructions

Authors:

Gardner et al

Abstract:

Analysis of dredged samples from Gulden Draak Knoll demonstrate it is a submarine rifted continental fragment that lies at the boundary between the western Perth Abyssal Plain and Wharton Basin, Indian Ocean. The Knoll comprises a granulite facies basement, including pelitic paragneiss and mafic orthogneiss, with a Cambrian granite inferred to intrude the other rocks. Boulders and cobbles of felsic gneiss with Mesoproterozoic and Cambrian protolith ages were also sampled likely reflecting a complex basement to variable sedimentary and volcanic rocks. The U-Pb isotopic system in Archean and Mesoproterozoic zircon is significantly disturbed, reflecting Cambrian orogenesis that affected all samples. The protolith to garnet-sillimanite-biotite paragneiss has a maximum deposition age of 1163 ± 24 Ma and includes older detrital zircon grains with populations at c. 2.65 Ga and between 1.4-1.1 Ga. A younger population in this sample is interpreted as a mix of newly grown metamorphic zircon and isotopically reset zircon, implying the granulite facies metamorphism occurred at c. 511 ± 5 Ma. Protracted Cambrian orogenesis is indicated by a metamorphic age in the mafic orthogneiss of 530 ± 6 Ma and isotopic disturbance shortly following emplacement of granite (c. 540 Ma with zircon ages disturbed to 509 ± 7 Ma) and the protolith to the felsic orthogneiss (c. 528 Ma with zircon ages disturbed to 510 ± 3 Ma). Xenocrystic zircon grains in Cambrian rocks include Archean (c. 2839 ± 9 Ma) and Mesoproterozoic (1230–1370 Ma) populations also isotopically disturbed during Cambrian orogenesis. Igneous Cambrian zircon grains have less radiogenic Hf-isotope compositions (Hfi = 0.281821-0.281367) than Mesoproterozoic xenocrysts (Hfi = 0.282267-0.281993), indicating limited involvement of the Mesoproterozoic crust in granite production. A more likely source includes Archean crust represented by xenocrysts with Hfi = 0.281399-0.280863. The Gulden Draak Knoll is reconstructed in Gondwana (‘Leeuwin’ full-fit model) along strike of a major structure termed the Indo- Australo- Antarctic Suture (IAAS), recently mapped from geophysical interpretations in Wilkes Land, Antarctica. New isotopic data suggest basement rocks from the Gulden Draak Knoll have affinity to crust exposed either side of the IAAS. Determining if this structure is a suture zone sensu stricto remains to be tested.

Absorption of the Tarim Craton into the PaleoProterozoic Supercontinent Columbia

Geochronology and geochemistry of meta-mafic dykes in the Quanji Massif, NW China: Paleoproterozoic evolution of the Tarim Craton and implications for the assembly of the Columbia supercontinent

Authors:

Liao et al

Abstract:

The Quanji Massif, located in the northeastern margin of the Tibet Plateau, is interpreted as a fragment of the Tarim Craton. The massif exposes a suite of metamorphosed mafic dykes. LA-ICP-MS U–Pb dating constrains the timing of intrusion of these dykes at ca. 1834 ± 23 Ma. The dykes display minor variations in major elements, with SiO2 = 46.8–53.4 wt%, MgO = 5.19–8.10 wt%, FeOt = 8.83–15.6 wt%, TiO2 = 0.58–1.78 wt% and Mg# = 46.3–69.7. Their immobile trace element compositions show a sub-alkali basalt affinity. The positive correlation of TiO2 with FeOt/MgO in these rocks shows an arc tholeiite evolutionary trend. The enrichment of LILE and LREE and depletion in HFSE suggest that the precursor magma was generated in a back-arc environment. Furthermore, these rocks possess (Nb/La)N of 0.31–0.52 and (Th/La)N of 0.61–1.39, whole rock (87Sr/86Sr)i values of 0.707598–0.724141, ɛNd(t) of −2.9 to +0.3 and (206Pb/204Pb)t of 16.8148–24.1513, (207Pb/204Pb)t of 15.3422–16.3630 and (208Pb/204Pb)t of 36.8415–39.7926. The magmatic zircons yielded ɛHf(t) of −3.4 to +6.1 and depleted mantle model ages (TDM) of 2.01–2.37 Ga. The geochemical and isotopic characteristics of these rocks suggest that their precursor magma was derived from a subduction-related fluid-metasomatized subcontinental lithospheric mantle mixed with a depleted mantle component. We suggest that a prolonged subduction-accretion-collision process along the southeastern margin of the Tarim Craton prevailed at ∼2.1–1.80 Ga, with local consumption of the Southeast Tarim Archipelagic Ocean during the collision of the Southeast Tarim Block with the Quanji Massif (microcontinent) at around 2.1–1.9 Ga. The final closure of the whole Southeast Tarim Archipelagic Ocean occurred at ∼1.85–1.82 Ga and collision between the Southeast margin of the Tarim Craton and other unknown continental blocks at 1.82–1.80 Ga, broadly coeval with the amalgamation of the Western and Eastern Blocks of the North China Craton and their assembly in the Columbia supercontinent.

Evidence of the Collison of the Precambrian Kokchetav Microcontinent With Future Asia During Paleozoic

Formation of the Kokchetav subduction-collision zone (northern Kazakhstan): Insights from zircon U-Pb and Lu-Hf isotope systematics

Authors:

Glorie et al

Abstract:

The Kokchetav subduction-collision zone is located in the western part of the ancestral Central Asian Orogenic Belt. This zone is built up by the Precambrian Kokchetav microcontinent which includes a HP-UHP metamorphic belt, and the North Kokchetav tectonic zone (NKTZ) which represents an accretionary complex between the Kokchetav microcontinent and the adjacent Stepnyak island-arc. The entire region is widely intruded by Palaeozoic granitoids which were emplaced after the collision of the Stepnyak island-arc with the Kokchetav microcontinent. We present zircon U-Pb ages and Lu-Hf systematics in zircon to better characterize the tectonic evolution of the Kokchetav subduction-collision zone.

The Lu-Hf results indicate that the Kokchetav basement rocks are derived from late Neoarchaean – early Palaeoproterozoic (~ 2.5 Ga) crust. For the granite-gneiss basement of the Kokchetav microcontinent, early Mesoproterozoic (Grenville-age, ~ 1.17-1.14 Ga) zircon U-Pb crystallization ages were obtained. For the NKTZ, two main age-components were recognized: (1) an oldest Mesoproterozoic age-component (~ 1.20-1.05 Ga) similar as for the Kokchetav microcontinental zircons, and (2) a younger Early Cambrian (540-520 Ma) or Late Cambrian - Early Ordovician (~ 490-480 Ma) age-component. Th/U ratios (< 0.1) are indicative of a metamorphic origin for both Early Cambrian and Late Cambrian - Early Ordovician zircon populations. The oscillatory zoned Mesoproterozoic zircons have higher Th/U ratios (> 0.1) and are typical of a magmatic protolith. The distinction between both zircon types is supported by characteristic cathodoluminescence images. These results confirm previous observations, indicating early Palaeozoic high-grade metamorphism (~ 540-520 Ma) and collisional deformation (~ 490-480 Ma) of the Mesoproterozoic Kokchetav zone as a result of subduction-accretion and collision of the Stepnyak island-arc to the Kokchetav microcontinent. For two additional samples of the Balkashin granitic complex Early Devonian (~ 415-395 Ma) zircon crystallization ages (magmatic Th/U ratios) were obtained. The occurrence of a Mesozoic xenocryst within these leucogranites may indicate that they were emplaced in a continental-arc setting.