Meteoritics & Planetary Science 40, Nr 6, 841–854 (2005) Abstract available online at http://meteoritics.org Argon isotopic analysis of breccia veins from the Roter Kamm crater, Namibia, and implications for their thermal history David RAJMON*, Peter COPELAND, and Arch M. REID Department of Geosciences, University of Houston, Houston, Texas 77204, USA *Corresponding author. E-mail: [email protected] (Received 02 April 2002; revision accepted 23 March 2005) Abstract–The rocks exposed in the rim of the 2.5-km-wide and 3.7-Ma-old Roter Kamm crater in southwest Namibia are cut by breccia veins that macroscopically resemble, and were originally described as, pseudotachylytes. The veins were later shown to be cataclasites with no evidence for melting. 40Ar/39Ar data for vein and host rock samples indicate a low-grade metamorphic event at around 300 Ma, but provide no evidence for an impact age. The samples have suffered 5–7% Ar loss, which we associate with the impact event. All the samples record similar ranges of possible time- temperature conditions and there are no resolvable differences between the results for the vein and the host rock samples, as would be expected if frictional heating played an important role in breccia formation. Modeling the 40Ar/39Ar data, assuming instantaneous impact heating followed by extended cooling, and coupling these results to published data on fluid inclusions, quartz precipitation, shock effects, and crater degradation, suggest that the veins reached maximum temperatures of 230–290 °C during impact and never approached melting temperatures of the precursor rocks. INTRODUCTION large impact craters (Reimold 1995, 1998; Spray 1998) and on major tectonic faults (e.g., Camacho et al. 1995). The chronology of impact events on Earth is important in Pseudotachylytes have been generated in shock experiments addressing various geologic problems, such as the role of (e.g., Fiske et al. 1995; Kenkmann et al. 2000) and in impacts in geologic evolution of the Earth (e.g., Dressler et al. experiments involving localized cataclasis and frictional 1994), the relationship between impacts and biologic and melting (Spray 1987). The term pseudotachylyte has been environmental evolution (e. g., Ryder et al. 1996; Vonhof often used in a much wider sense for similar-looking rocks, et al. 2000), reconstruction of the terrestrial meteorite flux including various types of tectonic and impact breccias, (e.g., Farley et al. 1998), and stratigraphic and reflecting limitations in our understanding of the genesis and paleogeographic reconstructions (e.g., Courtillot et al. 2000). the difficulty in proper recognition of these various rock types Isotopic ages of impact craters have been obtained both from (Reimold 1995, 1998). However, a misinterpretation of impact melt rocks and from pseudotachylytes (e.g., Deutsch unmelted breccias as pseudotachylyte can lead to erroneous and Sch‰rer 1994; Spray et al. 1995). Smaller impacts isotopic dating results. produce little if any melt but, in some cases, generate breccias Breccia veins occurring in the Roter Kamm crater of SW similar to pseudotachylyte (Reimold and Miller 1989; Namibia were originally described as pseudotachylytes Degenhardt et al. 1994). (Reimold and Miller 1989). Degenhardt et al. (1994) Pseudotachylyte is a very fine-grained, glass-like rock concluded that the veins did not actually contain melted that occurs as veins and irregular breccia matrices within host material but retained the term pseudotachylyte. We report rocks. The veins may appear intrusive into the host rock and here results of 40Ar/39Ar analysis of samples from the contain abundant clasts of the host rock. True Degenhardt study and interpret these data in terms of the pseudotachylytes show evidence of the former presence of a thermal history of the Roter Kamm breccia veins. The results melt (such as glass, microlites, vesicles, chilled margins; e.g. bear on general understanding of thermal effects of impacts Reimold 1998 and references therein) of frictional origin. on target rocks. While the thermal effects of large impact Pseudotachylytes have been found associated with several structures have been studied extensively (e.g., Staudacher 841 © The Meteoritical Society, 2005. Printed in USA. 842 D. Rajmon et. al. to about 35 GPa (Huffman and Reimold 1996; French 1998), were reported by Reimold and Miller (1989) and Degenhardt et al. (1994). Multiple sets of PDFs in quartz within suevite were also reported by Reimold et al. (1997). Shock features, reported from a single impact melt rock sample (Reimold and Miller 1989), include diaplectic quartz (corresponding to 35– 45 GPa according to compilation in French 1998, or > 27 GPa according to Huffman and Reimold 1996) and vesicular glass (∼45–50 GPa, French 1998). Besides the dated impact melt rock, melt was reported only in the form of several fragments in suevite (Reimold et al. 1997). Degenhardt et al. (1994) found that the pseutotachylyte- like breccias contained irregular clasts of the host rock displaying a wide range of grain sizes from several millimeters to about 10 µm. The finest grains displayed granoblastic texture suggestive of recrystallization. The dark color of the breccias was attributed to the presence of fine opaque minerals in quartz grains. No evidence for the presence of melt was found. According to these authors, the overall textural and geochemical characteristics of the veins suggested minimal transport of the vein material relative to the immediate host rock and minimal introduction of exotic Fig. 1. The location of the Roter Kamm crater. material. They concluded that the breccias were formed by local comminution followed by recrystallization of mainly et al. 1982; Boer et al. 1996; Moser 1997; Gibson et al. 1998; fine-grained material and that the heat generated during this Ivanov and Deutsch 1999), data on the thermal effects of process was insufficient to melt the rock. small impacts are rare (Koeberl et al. 1989). ANALYTICAL TECHNIQUES BACKGROUND Selected samples of vein and host rock material (Table 1, Roter Kamm is a simple crater, 2.5 km in diameter, Fig. 2) were crushed and processed by density and magnetic located in southwestern Namibia (27°46′S, 16°18′E, Fig. 1). separation in order to concentrate K-feldspar for 40Ar/39Ar Its impact origin has been established by a number of analyses. Composition of each concentrate was roughly geological, petrological (Reimold and Miller 1989; Reimold estimated based on XRD data and visual checking under a et al. 1997), and geophysical (Fudali 1973; Brandt et al. 1998) binocular microscope. The small grain sizes (∼1–200 µm in studies. Erosion has lowered the crater rim by 40–130 m and the vein matrix) made recognition and manual separation of most of the ejecta, apart from a few patchy occurrences, have different feldspars and other minerals difficult. Therefore, the been stripped away (Grant et al. 1997). 40Ar/39Ar analysis of K-feldspar concentrates contain some quartz and two of them an impactite sample provided an age for the impact event of also contain some albite. 3.7 ± 0.3 (1σ) Ma (Koeberl et al. 1993). Analytical and data reduction procedures for Ar isotopic The rocks exposed at the crater rim are gneisses and analyses follow those used by Spell et al. (1996) at the granites belonging to the Namaqualand Metamorphic University of Houston, except for a few differences. The K- Complex with a metamorphic age of 900–1200 Ma (Reimold feldspar concentrates were irradiated at a reactor at the and Miller 1989). At the time of impact, the Namaqualand University of Michigan together with fluence monitors gneisses were probably covered by a thin layer of Late (Australian National University 92-176, Fish Canyon Tuff Proterozoic Gariep metasedimentary rocks, which are sanidine, assumed age = 27.9 Ma, Steven et al. 1967; Cebula ∼700 Myr old. et al. 1986). Correction factors for reactions on Ca were (39Ar/ The breccias occur as numerous fine-grained dark- 37Ar) = 6.689 (±0.071) × 10−4 and (36Ar/37Ar) = 2.678 colored veins and dykes of submillimeter to meter widths. (±0.036) × 10−4, and for reactions on K (38Ar/39Ar) = 1.077 × The larger veins appear to extend radially from the crater, 10−2 and (40Ar/39Ar) = 4.950 (±0.090) × 10−2; discrimination whereas the fine veinlets may form networks without factor was 9.960 (±0.018) × 10−1 (all errors are 1σ). preferred orientation. The rocks show rare evidence for shock Irradiation factors with their respective errors are presented metamorphism. Planar deformation features (ω and π in together with the data in Table 2. The samples were heated by quartz), together indicating maximum shock pressures of 20 steps in a double-vacuum resistance furnace. Argon isotopic analysis of breccia veins from the Roter Kamm crater 843 Fig. 2. Hand specimen RK-SR and RK-1. RK-SR is granitic gneiss with a dark-to-greenish gray breccia vein. The host rock is dominated by pink microcline with smaller amount of albite and quartz, and is strongly fractured with many narrow breccia veinlets (indicated by arrow). The main vein displays a flow-like texture. The large fragments within the breccia match composition of the host gneiss. The network of the fine light fractures (indicated by arrows) was generated during sample manipulation. RK-1 is light gray granite cut by networks of narrow breccia veinlets. Locations, from which the analyzed K-feldspar concentrates were separated, are indicated. See text for further explanations. Table 1. List of samples. Hand specimen K-feldspar concentrate RK-SR Compact vein of dark breccia, 4–5 cm thick, in pink granitic gneiss. Host rock → RK-SR-VR Microcline > plagioclase, quartz, mica, a few percent of From fragment of the brecciated host rock adjacent to the main oxides, and minor secondary carbonate. Multiple narrow vein, fractures filled with narrow breccia veinlets.