Basic to Acid Pyroclasts Deposited in A Sediment-hosted Geothermal System in A Permian Lacustrine Rift Basin: Implications for Ancient Hydrovolcanic Sedimentation
Abstract
The sedimentation of pyroclasts formed by sublacustrine volcanic and hydrothermal processes are complex and affected by physical, chemical and biological factors. Once the clasts mixed with lacustrine sediments, they are hardly to be identified and researched. We propose four groups of pyroclasts dominated rocks which are intercalated within fine-grained sedimentary rocks (mainly interlaminated tuffaceous shales with dolomicrite) in Permian Lucaogou Formation in lacustrine Santanghu rift basin, Xinjiang, NW China. Mineralogy and petrology were studied on samples from cores and outcrop to reflect the sedimentary processes and sources. Four lithofacies (figure 1) suggesting basic to acid volcanic origins were observed. They are combinations of magmatic, hydrothermal, and sedimentary minerals, such as dolomite-smectite, diopside- wollastonite, analcime- peralkaline feldspars, and felsic grains-carbonate cements. The former two are thin bedded to laminated with sharp boundaries with adjacent shales. Their grains are massive, poorly sorted, angular, and supported by matrices, indicating a high-density granular deposition originated from basic and peralkaline-alkaline carbonatite and pyroxenite. The latter two are laminated with a sharp bottom and gradually top with adjacent dolomicrite. Their grains are normal graded, moderately sorted, angular (some of them are shard-like), and cemented by micritic dolomite, indicating a turbidity deposition originated from medium to acid analcime phonolite and andesite. These grains have low compositional and textural maturity, which are not common in traditional lacustrine fine-grained sedimentary rocks. Embayed boundaries are common in individual mineral as well as aggregated grains, suggesting multiple episodes of melting and welding under a high temperature condition. These lithofacies are interpreted as sublacustrine hydrovolcanic deposits at the sediment-hosted geothermal system, where the pyroclasts had been altered by syndepositional hydrothermal fluids and mixed with lake deposits. The interpretation is substantiated by abundant cone-shaped stratigraphic buildups on seismic sections in the basin. The different property of volcanic sources may be caused by various depth of volcanisms in a rift basin.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.V43G0175M
- Keywords:
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- 8408 Volcano/climate interactions;
- VOLCANOLOGY;
- 8414 Eruption mechanisms and flow emplacement;
- VOLCANOLOGY;
- 8424 Hydrothermal systems;
- VOLCANOLOGY;
- 8427 Subaqueous volcanism;
- VOLCANOLOGY