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Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28r.408n&link_type=abstract
Meteoritics, vol. 28, no. 3, volume 28, page 408
Other
7
Hydrothermal Alteration, Impact Craters, Puchezh-Katunki Astrobleme
Scientific paper
The giant (D = 80 km) Puchezh-Katunki astrobleme [1,2] is the site of widespread hydrothermal alteration. It occurs mainly in the central uplift composed of brecciated archean rocks and overlain by allogenic breccia, suevites, and coptomict gravelites (the latter is the lowest bed of crater-lake deposits). Distribution and vertical zonation of secondary minerals is controlled by the thermal gradient during cooling of authigenic breccia massif, while the degree of alteration depends on the intensity of brecciation and fracturing of basement rocks. Three types of hydrothermal mineralization are distinguished on the basis of different crystallization conditions: (1) mixed hydrothermal-diagenetic, in coptomict gravelites; (2) metasomatic, including formation of Fe-Mg hydrous phyllosilicates in shocked and thermally altered gneisses and amphibolites; and (3) veins, represented by mineral associations filling fractures and voids in basement rocks, allogenic breccia, and suevites. The second and third types occur together and each displays characteristic zonations, thus several zones may be distinguished in vertical section (zones 2-4 below). The uppermost zone corresponds to mixed hydrothermal-diagenetic conditions described in (1) above. In general, four zones are distinguished, from top downward, as follows. 1. Zone of hydrothermal-diagenetic alteration in coptomict gravelites. In this zone, replacement of impact glass fragments by assemblage of montmorillonite, calcite, and pyrite, and formation of alkali zeolites and calcite as a cement are observed establishing a temperature of alteration of less than 100 degrees C. 2. Zone of low-temperature (100 degrees-200 degrees C) mineralization comosed of suevites, allogenic breccia, and the upper part of authigenic breccia down to 2.5 km. Fe-saponite develops in shocked and recrystallized basement rocks, and various zeolites, apophylite, calcite, anhydrite, and pyrite fill vugs and fractures; in addition, calcite-nontronite veinlets occur locally. The distribution of zeolites is characterized by their own zonation [3]. 3. Zone of moderate-temperature (200 degrees-300 degrees C) mineralization in basement at a depth of 2.5-4.2 km. Chlorite (of diabantite-pictochlorite series) is a common metasomatic phase associated with pyrite, and also with albite, epidote, and calcite locally. In veinlets, Ca-Fe silicates (andradite, salite, epidote, prehnite) together with pyrite, chlorite, and, very rarely, quartz, are found in a laumontite-anhydrite matrix. 4. Zone of low-moderate-temperature (150 degrees - 250 degrees C?) mineralization at a depth below 4.2 km. Prehnite, anhydrite, calcite, and pyrite are present. Absence of Ca-Fe silicates may indicate a decrease in crystallization temperature compared with the upper zone, while disappearance of hydrous phases seems to be a result of the fall of P(sub)H2O at this depth. The decrease in degree of alteration outward from the impact center and the change in zeolite composition to lower-temperature varieties as well as crystallization of gypsum instead of anhydrite are evidence of lateral hydrothermal zonation in the crater. Thus the zones may have an ellipsoidal shape corresponding to isotherms during circulation. The generalized chronological order of hydrothermal crystallization is the following: Ca-Fe silicates, chlorite (smectites)-calcite, pyrite-anhydrite- zeolites, apophyllite-calcite, nontronite. This reflects a continually decreasing temperature during the process of hydrothermal alteration. Spatial segregation of minerals is caused by change of parameters (pH, f(sub)CO2, temperature, etc.) of solutions moving in the thermogradient field, where the source of heat is a result of the thermal history of the central uplift [4]. Nevertheless, the role of endogenic heat and mass transfer in impact-induced hydrothermal circulation processes cannot be ruled out. References: [1] Masaitis V. L. and Mashchak M. S. (1990) Meteoritics, 25, 383a. [2] Pevzner L. A. et al. (1992) LPS XXIII, 1063-1064. [3] Naumov M. V. (1992) LPS XXIII, 967-968. [4] Masaitis V. L. and Naumov M. V, this issue.
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