The standard interpretation of textures in igneous rocks was largely set over a century ago, when pioneering petrographers such as H.C. Sorby (1858) and L.V. Pirsson (1909) systematized the study o...
Abstract The geology of the 2.7 Ga Stillwater Complex in South-Central Montana is reviewed with a focus on the role of volatiles in locally modifying both the crystallization sequence of the evolving parent magma and the initially precipitated solid assemblages to favour olivine ± chromite. A secondary origin for these two minerals is particularly probable for the olivine-bearing rocks of the Banded series and, at a minimum, also increasing their modal abundance in the Peridotite zone of the Ultramafic series. Direct evidence for volatiles includes the presence of high-temperature fluid inclusions in pegmatoids and hydrous melt inclusions (now crystallized) in chromite and olivine from both the Ultramafic and the Banded series rocks. Indirect evidence includes the boninitic character of the parent magma, the presence of volatile-bearing minerals including high-temperature carbonates, rock textures, and Cl / F variations in apatite. Mechanisms which favour the formation of olivine (± chromite) over pyroxene include volatile phase boundary shifts induced by added H 2 O, incongruent melting of pyroxene by hydration of a partly-molten mush, and the near- to sub-solidus replacement of pyroxene by olivine and chromite by silica-undersaturated fluids. These mechanisms cast doubt that magmas with different liquid lines of descent were involved in the crystallization of the Stillwater Complex. A dry Stillwater magma would have been mineralogically and modally much less varied and lacking in high-grade platinum-group element and chromium deposits.
In their discussion of our recent publication, Scoon and Mitchell (2020 Scoon, R., and Mitchell, A., 2020, Discussion of “Crustal fluid contamination in the Bushveld Complex, South Africa: An analogue for subduction zone fluid migration “by Benson, E., Connolly, J.A.D. and Boudreau, A.E. (2020): International Geology Review.[Taylor & Francis Online], [Web of Science ®] , [Google Scholar]) put forward a number of arguments against the hydromagmatic model of Bushveld Complex formation that we present. Their criticisms of our model focus primarily on the formation mechanisms of the discordant bodies present at Bushveld, namely the iron-rich ultramafic pegmatites and the dunite pipes. While this was a minor portion of our paper, we here review evidence in favor of a fluid-related origin for these discordant bodies, in contrast to the primarily magmatic origin that Scoon and Mitchell present.
Research Article| October 19, 2017 The origin of high-Cu/S sulfides by shallow-level degassing in the Skaergaard intrusion, East Greenland Chao Li; Chao Li 1Division of Earth and Ocean Sciences, Box 90227, Nicholas School of the Environment, Duke University, Durham, North Carolina 27708, USA Search for other works by this author on: GSW Google Scholar Alan E. Boudreau Alan E. Boudreau 1Division of Earth and Ocean Sciences, Box 90227, Nicholas School of the Environment, Duke University, Durham, North Carolina 27708, USA Search for other works by this author on: GSW Google Scholar Geology (2017) 45 (12): 1075–1078. https://doi.org/10.1130/G39540.1 Article history received: 19 Jul 2017 rev-recd: 31 Aug 2017 accepted: 05 Sep 2017 first online: 19 Oct 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Tools Icon Tools Get Permissions Search Site Citation Chao Li, Alan E. Boudreau; The origin of high-Cu/S sulfides by shallow-level degassing in the Skaergaard intrusion, East Greenland. Geology 2017;; 45 (12): 1075–1078. doi: https://doi.org/10.1130/G39540.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search nav search search input Search input auto suggest search filter All ContentBy SocietyGeology Search Advanced Search Abstract The Skaergaard intrusion of East Greenland is characterized by unusually low sulfur (S) and high metal/S ratios. Explanations range from S lost during cooling to suggestions that low S was characteristic of the original magma. A nonequilibrium thermodynamic analysis of modal variations in the chilled margin and Lower Zone is consistent with a late stage reaction of chalcopyrite → magnetite ± bornite + Slost; the pre-reaction assemblage was dominated by chalcopyrite with a Cu:S mass ratio ∼1:1, a ratio unusually low for an undegassed hotspot basalt. A comparison with other hotspot and flood basalt provinces suggests that these magmas can lose significant S to recirculating shallow-level degassed magma. It is concluded that degassing of the parent Skaergaard magma prior to solidification and continuous vapor loss during crystallization both contributed to the overall high metal/S ratios of the Skaergaard system. You do not currently have access to this article.
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Abstract The presence of pegmatoid bodies in the Stillwater Complex is poorly understood, but they have been suggested to have resulted from the presence of fluids in the complex. To better understand the origin of the pegmatoids and to trace the possible influence of country-rock-derived fluid in the Stillwater Complex, bulk rock Rb-Sr, Sm-Nd, and Pb-Pb isotopes for samples from the Archean Stillwater Complex and its metamorphic aureole are reported. Pegmatoid bodies are compared to spatially associated host rock and the underlying hornfels facies country rocks. Evidence of resetting of radiogenic isotopes during regional metamorphism at 1700 Ma is not observed, and the initial radiogenic isotopic ratios in Stillwater Complex rocks overlap those of the underlying hornfels. Despite the isotopic similarity of the country rock to the Stillwater Complex, the intrusion is modestly isotopically heterogeneous. In Stillwater samples, the average εNd,2710Ma = −1.1 ± 6.9, 206Pb/204Pb2710 Ma = 15.24 ± 2.26, and 87Sr/86Sr2710Ma = 0.703043 ± 0.002747 (1σ). The similarity between country rock and intrusive rock isotopic compositions at Stillwater contrasts with the data reported for the Bushveld Complex, South Africa, where the country rock is isotopically distinct from the intrusion. The variability in radiogenic isotope signatures in Stillwater rocks show a noisy but decreasing influence of country rock up through the Lower Banded series interpreted to reflect variable crustal contamination, in part from <1.0 wt % country rock fluids released during intrusion of the Stillwater Complex. The influence of crustal fluid contamination as compared to more traditional crustal assimilation models or simple magmatic heterogeneity suggests that hydrothermal fluids modified the isotopic compositions of more fluid-mobile elements and can explain aspects of isotopic heterogeneity in layered intrusions.
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