On the depth of crater excavation and melting during the Sudbury impact: geochemical evidence from chilled impact melt dykes

2020 ◽  
Author(s):  
Alexander Kawohl ◽  
Hartwig E. Frimmel ◽  
Wesley E. Whymark ◽  
Andrejs Bite
Keyword(s):  
Continental Crust ◽  
Bulk Composition ◽  
Geochemical Data ◽  
Tectonic Deformation ◽  
Target Area ◽  
Upper Crust ◽  
Igneous Complex ◽  
Impact Melt ◽  
Sudbury Igneous Complex ◽  
The Impact

<p>The 1.85 Ga Sudbury Igneous Complex, Canada, is the remnant of a ~3 km thick impact-generated crustal melt sheet, caused by a 10-15 km large chondritic asteroid or comet that had left behind an impact structure of ~200 km prior to tectonic deformation und subsequent erosion. However, less is known about how deep the impactor penetrated the continental crust and where the source of the impact melt was. Mixing models including radioisotopes and trace elements on locally exposed country rocks have been used to evaluate their relative contribution to the impact melt. Based on this, Darling et al. (2010) have argued for shallow melting of the upper crust (UCC) only, either due to an oblique impact and/or a low-density bolide (comet). In contrast, the abundance of siderophile elements in impact melt-rocks was taken as evidence of a lower crustal source (Mungall et al. 2004), i.e. overlying rocks of the middle and upper crust must have been removed during the crater excavation stage. U-Pb age data on zircon xenocrysts also point to the involvement of rock types not exposed on surface (Petrus et al. 2016) in agreement with theoretical simulations, which have predicted a >20 km deep but unstable transient cavity (Ivanov & Deutsch 1999).</p><p>Large-scale (10s of km) and well-exposed impact melt dykes are a unique feature of Sudbury. The dykes are of granodioritic/quartz dioritic composition and are interpreted as clast-laden melt injections into the basement instantaneously after the impact (Pilles et al. 2018). Their vitric margins and distal extremities should therefore approximate the undifferentiated bulk composition of the Sudbury Igneous Complex prior to sulfide saturation. A compilation of published and new geochemical data of these dykes reveal a remarkably strong affinity (r<sup>2</sup> >0.989) to the average middle continental crust (MCC) as given by Rudnick & Gao (2014), especially in terms of major elements and fluid-immobile transition metals (Th, Zr, Hf, Nb, Ta, Ti, Sc, REE). The dykes are, however, significantly enriched in Ni, Cu and Cr, and to a lesser extent in V, Co and P relative to the typical UCC and MCC. A systematic loss of volatiles (Tl, Cd, Sn, Zn, Pb, Ag, Cs, Rb, Na, K, Ga, As) compared to either crustal model is not evident. These new observations favour a scenario in which the impactor and supracrustal rocks in the target area became vaporized and ejected. Shock melting affected predominantly the middle crust of the Canadian Shield. We also propose that the rocks that contributed to the impact melt were, on average, more mafic than the typical UCC and MCC. This is consistent with the report of exotic mafic-ultramafic xenoliths within the Sudbury Igneous Complex (Wang et al. 2018) and its anomalously high PGE concentrations (Mungall et al. 2004). (Ultra-)mafic rocks hidden at mid-crustal depth were a likely source of Ni-Cu-PGE-Co and gave rise to world class ore deposits presently mined at Sudbury. Such (ultra-)mafic intrabasement body might also explain the 1200 km<sup>2</sup> Temagami magnetic anomaly in the eastern vicinity of the Sudbury Complex.</p>

New insights into the structure of the Sudbury Igneous Complex from downhole seismic studies

2002 ◽  
Vol 39 (6) ◽  
pp. 943-951 ◽  
Author(s):  
David Snyder ◽  
Gervais Perron ◽  
Karen Pflug ◽  
Kevin Stevens
Keyword(s):  
Seismic Data ◽  
Seismic Profile ◽  
Seismic Profiles ◽  
Igneous Complex ◽  
Impact Melt ◽  
Vertical Seismic Profiles ◽  
Sudbury Igneous Complex ◽  
Common Depth Point ◽  
Deformation Models ◽  
The Impact

New vertical seismic profiles from the northwest margin of the Sudbury impact structure provide details of structural geometries within the lower impact melt sheet (usually called the Sudbury Igneous Complex) and the sublayer norite layer. Vertical seismic profile sections and common depth point transformation images display several continuous reflections that correlate with faults and stratigraphic boundaries logged from drill cores. Of four possible mechanisms that explain repeated rock units, late-stage flow or normal faulting that occurred within the last layers to cool and crystallize might best explain the observations, especially the most prominent reflectors observed in the seismic data. These results reaffirm previously proposed two-stage cooling and deformation models for the impact melt sheet.

scholarly journals Geochemistry and Petrogenesis of Mafic and Ultramafic Inclusions in Sublayer and Offset Dikes, Sudbury Igneous Complex, Canada

2020 ◽  
Vol 61 (6) ◽  
Author(s):  
Yujian Wang ◽  
C Michael Lesher ◽  
Peter C Lightfoot ◽  
Edward F Pattison ◽  
J Paul Golightly
Keyword(s):  
Matrix Group ◽  
Igneous Complex ◽  
Middle Crust ◽  
Group Iii ◽  
Impact Melt ◽  
Sudbury Igneous Complex ◽  
The North ◽  
Group I ◽  
Group Ii ◽  
The Impact

Abstract The c. 1·85 Ga Sudbury Igneous Complex (SIC) is the igneous remnant of one of the oldest, largest and best-preserved impact structures on Earth and contains some of the world’s largest magmatic Ni–Cu–PGE sulfide deposits. Most of the mineralization occurs in Sublayer, Footwall Breccia and inclusion-bearing quartz diorite (IQD), all of which contain significant (Sublayer and IQD) to minor (Footwall Breccia) amounts of olivine-bearing mafic–ultramafic inclusions. These inclusions have only rare equivalents in the country rocks and are closely associated with the Ni–Cu–PGE sulfide mineralization. They can be divided into three groups on the basis of petrography and geochemical characteristics. Group I (n = 47) includes igneous-textured olivine melanorite and olivine melagabbronorite inclusions in the Whistle and Levack embayments on the North Range with Zr/Y, Zr/Nb, Nb/U and Zr/Hf similar to igneous-textured Sublayer matrix. Group I inclusions are interpreted to be anteliths that crystallized from a mixture of SIC impact melt and a more mafic melt, probably derived by melting of ultramafic footwall rocks. Group II includes Group IIA (n = 17) shock metamorphosed wehrlite and olivine clinopyroxenite inclusions in the Levack embayment and Group IIB (n = 2) shock metamorphosed olivine melanorite inclusions in the Foy Offset on the North Range. Group II inclusions have similar trace element patterns [e.g. negative Th–U, Nb–Ta–(Ti), Sr and Zr–Hf anomalies] and overlapping Nb/U to a layered mafic–ultramafic intrusion in the footwall of the Levack and Fraser deposits, which together with their limited distribution suggests that Group II inclusions are locally-derived xenoliths. Group III (n = 21) includes phlogopite lherzolite and feldspar lherzolite inclusions with igneous, recrystallized and shock-metamorphic textures in the Trill, Levack and Bowell embayments, and the Foy Offset dike on the North Range. They have no equivalents in the exposed country rocks. The calculated parental magma is similar to continental arc basalt formed by approximately 5% partial melting of garnet peridotite. Ol–Cpx–Pl thermobarometry of several Group III inclusions indicate equilibration at 900–1120 ºC and 210 ± 166 MPa to 300 ± 178 MPa, suggesting crystallization in the upper-middle crust (7·7 ± 6·6 to 10·9 ± 6·5 km), prior to being incorporated into the lower parts of the impact melt sheet during impact excavation. The exotic xenoliths provide information about the depth of impact and composition of upper-middle crust in the Sudbury region at 1850 Ma, the local xenoliths provide information about the thermomechanical erosion process that followed generation of the impact melt, the anteliths provide information about the early crystallization history of the SIC, and all of the inclusions provide constraints on the genesis of Sublayer, IQD, footwall breccia, and associated Ni–Cu–PGE mineralization.

Evidences for Bimodal volcanism on 4-Vesta Asteroid from impact-melt clast

2020 ◽  
Author(s):  
Dwijesh Ray ◽  
Sambhunath Ghosh
Keyword(s):  
Melt Inclusions ◽  
Bulk Composition ◽  
Parent Body ◽  
Total Iron ◽  
Volcanic Complex ◽  
Element Geochemistry ◽  
Silicic Volcanism ◽  
Impact Melt ◽  
Acid Volcanism ◽  
The Impact

<p>Silicic / acid volcanism has not been widely described either on Moon, Mars or in Asteroid 4 Vesta. The occurrence of sialic crustal rocks on the lunar surface is extremely limited. Reports on silicic (non-mare) volcanic rocks on Moon is found to be associated in Compton-Belkovich volcanic complex, Hansteen Alpha volcanic crater, Lassell massif, Gruithuisen domes and ejecta of Aristarchus crater (Clegg-Watkins et al., 2017). The occurrence of several volcanic constructs (e.g. collapse features, domes) and volatile-rich pyroclastics in association with silicic rocks further emphasize existence of viscous magmas on Moon. A localized occurrence of silicic volcanism on Mars is also envisaged by the presence of tridymite in mudstone of Gale crater (Morris et al., 2016). However, the exact formation mechanism of silicic volcanism on Moon, Mars or even in 4-Vesta has been largely hindered due to lack of silicic meteorite samples or mission-returned samples.</p> <p>The HED (Howardite, Eucrite, Diogenite) meteorites is considered to have originated from a common parent body Asteroid 4-Vesta. Recent Dawn mission also attempts to validate its geologic context and formulate a possible HED-Vesta connection (McSween et al., 2013). Based on Dawn findings, Vesta’s surface appears to be similar to a mixture of basaltic eucrite and diogenite resembling a more complex breccia howardite (De Sanctis et al., 2012; Prettyman et al., 2012). A variety of clasts are apparently common in howardite. Here, we report the petrography and major element geochemistry of a new impact-melt clast from Lohawat howardite. Our results show that the clast composition is unique and unlikely to be explained by typical impact melting of HED mafic lithologies. One of the impact melts (~20µ across) hosted in ferroaugite (Wo<sub>42</sub>En<sub>2.7</sub>Fs<sub>55.3</sub>) clast substantially differ in composition from the other impact-melt (~50µ across) hosted in ilmenite clast, specially in terms of SiO<sub>2</sub> wt%, CaO wt%, K<sub>2</sub>O wt% and K<sub>2</sub>O / (K<sub>2</sub>O + Na<sub>2</sub>O) ratio. Moreover, one appears nearly homogeneous in contrast to evolved nature with limited heterogeneity as compared to other. Both the melts are oblong-shaped, smooth textured with sharp outline and embedded in the host monomict mineral clast of different composition belonging to possible parent of cumulate eucrite.</p> <p>The average bulk composition of Lohawat is consistent with basaltic crusts (SiO<sub>2</sub> ~50.3-51.8 wt%, Al<sub>2</sub>O<sub>3</sub> ~3.5-8.2 wt%, total iron-magnesia ~31.2-38.0 wt%, CaO ~2.2-7.6 wt%) (Chattopadhyay et al. 1998; Sisodia et al. 2001; Ghosh, 2011). Supplement to basaltic volcanism, we report for the first time the incipient acid volcanism in a HED meteorite based on two impact melt inclusions of nearly rhyolitic composition (SiO<sub>2</sub> ~76-79.5 wt%, Al<sub>2</sub>O<sub>3 </sub>~11.4 - 12.8 wt%, total alkali ~3 - 8 wt% with K<sub>2</sub>O/ (Na<sub>2</sub>O + K<sub>2</sub>O) ~0.21-0.95, CaO ~ 0.8 - 4.67wt% and low total iron-magnesia ~1-2 wt%). Our study thus reinforces to conceive the idea that some rhyolitic crusts formed due to differentiation of mafic magma were exposed on Vesta and heterogeneity of Vestan surface is definitely different from one as previously thought.</p> <p>References: Clegg-Watkins, R.N. et al. 2016, Icarus 285:169-184. Morris, R.V. et al. 2016, 113:7071-7076. McSween, H.Y. et al. 2013, MAPS 48:2090-2104. De Sanctis, M.C. et al. 2012, Science 336:697-700. Prettyman, T.H. et al. 2012, Science 338:242-246. Chattopadhyay, B. et al. 1998. JGSI 51:171-174. Sisodia, M.S. et al. 2001 MAPS 36:1457-1466. Ghosh, S. IJG 65:251-264.</p>

The Sudbury Igneous Complex: A Differentiated Impact Melt Sheet

2002 ◽  
Vol 97 (7) ◽  
pp. 1521-1540 ◽  
Author(s):  
A. M. Therriault ◽  
A. D. Fowler ◽  
R. A. F. Grieve
Keyword(s):  
Igneous Complex ◽  
Impact Melt ◽  
Sudbury Igneous Complex

High-Grade Magmatic Platinum Group Element-Cu(-Ni) Sulfide Mineralization Associated with the Rathbun Offset Dike of the Sudbury Igneous Complex (Ontario, Canada)

2020 ◽  
Vol 115 (3) ◽  
pp. 505-525
Author(s):  
Alexander Kawohl ◽  
Wesley E. Whymark ◽  
Andrejs Bite ◽  
Hartwig E. Frimmel
Keyword(s):  
Base Metal ◽  
Precious Metal ◽  
Quartz Diorite ◽  
Metal Sulfides ◽  
High Grade ◽  
Igneous Complex ◽  
Impact Melt ◽  
Sudbury Igneous Complex ◽  
Base Metal Sulfides ◽  
Platinum Group

Abstract Quartz dioritic impact melt dikes around the 1.85 Ga Sudbury Igneous Complex, locally referred to as offset dikes, are well endowed with respect to Ni-Cu-platinum group elements (PGE). However, only those dikes proximal (<6 km) to the main mass of the Sudbury Complex are mineralized at an economic grade and, in places, host world-class deposits. We report on a new discovery of such heavily mineralized offset dike at Rathbun Lake, about 15 km east of the currently known extent of the Sudbury Igneous Complex. There, a segment of amphibole quartz diorite is exposed at the contact between Huronian metasedimentary rocks and gabbro of the 2.22 Ga Nipissing Suite, xenoliths of which are abundant throughout the diorite and record textural evidence of partial melting. The mafic inclusion-bearing quartz diorite is the host of the Rathbun Lake showing, a small but high-grade PGE-Cu(-Ni) sulfide occurrence of hitherto controversial origin. A detailed petrographic and mineralogical characterization of this occurrence revealed a two-stage mineralization history. Disseminated to semimassive (net-textured) chalcopyrite ± loop-textured pentlandite ± magnetite containing Pd-bismuthotellurides and, more rarely, sperrylite and native gold—all of which are closely associated with base metal sulfides—are interpreted as magmatic. The semimassive sulfide averages ~40 g/t Pd + Pt + Au at a Cu/(Cu + Ni) of >0.9 and a Pd/Ir of >100,000. Mineralogy, ore textures, and mantle-normalized PGE + Au patterns match a specific type of Cu-rich mineralization in the Sudbury Igneous Complex known as footwall mineralization. By analogy with these footwall deposits, the occurrence is interpreted as having formed by downward percolation of a highly fractionated sulfide melt toward the bottom of a now largely eroded offset dike. The magmatic paragenesis was hydrothermally overprinted at lower greenschist-facies conditions to pyrite-chalcopyrite-violarite ± covellite ± millerite. This involved also local remobilization into pyrite-chalcopyrite veinlets and the liberation of precious metal minerals from their sulfide hosts. In contrast to base metal sulfides, most precious metal minerals were resistant to hydrothermal alteration, although corrosion of some grains is noted as well as their truncation by chlorite and epidote. Micron-scale X-ray mapping revealed a progressive replacement of magmatic Pd-Bi-Te minerals, where in contact with hydrous silicates, by Sb- and Hg-bearing Pd minerals such as temagamite, Pd3HgTe3. The timing and nature of this hydrothermal overprint remains uncertain, but a connection to later regional metamorphism and faulting seems most plausible. Our finding of magmatic PGE-base metal sulfide at Rathbun Lake suggests a new subtype of distal offset dike-hosted mineralization in an area so far not known for offset dikes. It opens up new opportunities in the search for unconventional ore deposits around the Sudbury impact structure and improves our understanding on the distribution of impact melt-derived dikes around complex craters.

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