Tectonically driven dolomitization of Cambrian to Lower Ordovician carbonates of the Quruqtagh area, north-eastern flank of Tarim Basin, north-west China

Dolomites occur extensively in Cambrian to Lower Ordovician carbonates in the Tienshan orogen of the Quruqtagh area, north-east Tarim Basin, where thick (up to 1 km), dark grey lenticular limestones of semi-pelagic to pelagic origin are prominent. The dolomites generally occur as beige, anastomosed geobodies that cross-cut well-stratified limestones. Based on detailed field investigations and petrographic examination, three types of matrix dolomite are identified: fine crystalline planar-e (Md1), fine to medium crystalline planar-s (e) (Md2) and fine to coarse crystalline non-planar-a (Md3) dolomites. One type of cement dolomite, the non-planar saddle dolomite (Cd), is also common. The preferential occurrence of Md1 along low-amplitude stylolites points to a causal link to pressure dissolution by which minor Mg ions were probably released for replacive dolomitization during shallow burial compaction. Type Md2, Md3 and Cd dolomites, commonly co-occurring within the fractured zones, have large overlaps in isotopic composition with that of host limestone, implying that dolomitizing fluids inherited their composition from remnant pore fluids or were buffered by the formation water of host limestones through water-rock interaction. However, the lower delta O-18 and higher Sr-87/Sr-86 ratios of these dolomites also suggest more intense fluid-rock interaction at elevated temperature and inputs of Mg and radiogenic Sr from the host limestones with more argillaceous matter and possibly underlying Neoproterozoic siliciclastic strata. Secondary tensional faults and fractures within a compressional tectonic regime were probably important conduits through which higher-temperature Mg-rich fluids that had been expelled from depth were driven by enhanced tectonic compression and heating during block overthrusting, forming irregular networks of dolomitized bodies enclosed within the host limestones. This scenario probably took place during the Late Hercynian orogeny, as the Tarim block collided with Tienshan island arc system to the north and north-east. Subsequent downward recharges of meteoric fluids into the dolomitizing aquifer probably terminated dolomitization as a result of final closure of the South Tienshan Ocean (or Palaeo-Asian Ocean) and significant tectonic uplift of the Tienshan orogen. This study demonstrates the constructive role of notably tensional (or transtensional) faulting/fracturing in channelling fluids upward as a result of intense tectonic compression and heating along overthrust planes on the convergent plate margin; however, a relatively short-lived, low fluid flux may have limited the dolomitization exclusively within the fractured/faulted limestones in the overthrust sheets.