Laser Ablated U-Pb Dating-Based Determination Of Burial Dolomitization Process : A Case Study Of Lower Ordovician Penglaiba Formation Of Yonganba Outcrop In Tarim Basin

Burial dolomitization is the major mechanism for the formation of massive dolomite, but its process has been of a dispute for a long time. In this paper, we took the Lower Ordovician Penglaiba Formation outcropped at Yonganba profile in Tarim Basin as an example, and studied the timing and process of dolomitization of the Penglaiba Formation by using laser ablated U-Pb dating and isotope analysis on the basis of detailed observation of the outcrop and thin sections of collected samples from this profile. It's concluded that: (1) the Penglaiba Formation consists of four types of dolomite, including microbial dolomite, euhedral to subhedral fine to medium crystalline dolomite with zonation under CL, enhedral medium crystalline dolomite with cloud core and bright edges, and anhedral coarse crystalline dolomite, and these four types of dolomites are interbedded vertically in the profile; (2) U-Pb dating revealed that three stages of dolomitization, including penecontemporaneous dolomitization, shallow burial dolomitization during Late Ordovician to Silurian (464 +/- 12Ma similar to 433 +/- 22Ma), and burial dolomitization during Devonian (382 +/- 29Ma). Shallow burial dolomitization tends to cause reclystallization of penecontemporaneous dolomite, and burial dolomitization leads to the overgrowth of pre-existing dolomite in certain degree. Additionally, involvement of Cambrian dolomitized fluids probably exist; (3) it's proposed that the formation of massive dolomite is controlled by depositional environment and tectonic-burial evolution, and as a result, the north slope of Tazhong uplift and the south slope of Tabei uplift could be favorable to the development of massive dolomite, which is meaningful to guide the hydrocarbon exploration.