Mechanisms for Regional Dolomitization of the Triassic Yangtze Platform, Guanling Area, in the Nanpanjiang Basin of South China
Date of Award
Thesis campus only
Daniel J Lehrmann
Lower to Middle Triassic strata of the Yangtze Platform contain a dolomitized interior, undolomitized boundstone margin, and partially dolomitized slope facies. Anhedral to subhedral dolomite replaces micrite, carbonate grains, and intergranular sparry calcite. Coarse euhedral dolomite cement lines depositional cavities. Vugs and early fractures contain saddle dolomite, while later fractures are filled with calcite. Stylolites crosscut most fractures and all dolomite phases.
Burial history predicts a maximum temperature of 180°C at 5 km depth at the base of the platform and a maximum temperature of 96°C at 2.2 km depth at the top of the platform, using regional stratal thickness in conjunction with a thermal gradient of 30°C/km.
Stable isotope measurements within dolomite show δ18O values ranging from -7.7‰ to 0.8‰ (VPDB) and δ13C values ranging from 0.77‰ to 4.0‰ (VPDB). Vein calcite isotope values range from δ18O -18.4‰ to -5.2‰ and δ13C -6.1 to 3.4‰. 87Sr/86Sr ratios from 11 of 14 dolomite samples showed values ranging from 0.707677 to 0.708601, consistent with Triassic seawater, while 2 samples showed elevated 87Sr/86Sr values, and 1 sample showed depleted 87Sr/86Sr values.
Average homogenization temperatures (Th) and freezing point depressions (Tmice) from 35 euhedral dolomite crystals in the platform interior, slope, and basin indicate entrapment of saline brines (9.5 to 15 wt. % NaCl) over temperatures of 113-150°C.
87Sr/86Sr ratios of dolomite are consistent with modified seawater including radiogenic contribution of hydrothermal fluids. The range in δ18O within dolomite is consistent with enrichment by evaporative concentration of seawater, but also includes negative values consistent with high temperature fluids.
The spatial distribution of dolomite indicates that early marine cementation of the margin prevented permeation of dolomitizing fluids. Hypersaline reflux dolomitization remains a possibility for platform interior, but the partial dolomitization of the slope and basin pelagic carbonates and geothermometry are more consistent with late hydrothermal dolomitization. The late diagenetic stage, presence of saddle dolomite, and geothermometry from over 90 to 185°C point to burial dolomitization.
Ledbetter Ferrill, Nathaniel S., "Mechanisms for Regional Dolomitization of the Triassic Yangtze Platform, Guanling Area, in the Nanpanjiang Basin of South China" (2023). Geosciences Student Honors Theses. 25.