Mg, C and O isotopic compositions of Late Cretaceous lacustrine dolomite and travertine in the northern Tianshan Mountains, Northwest China

Abstract

The Junggar Basin was characterized by high temperature and strong tectonic activities during the Late Cretaceous, in which a dolomite-forming lacustrine depositional system associated with travertine was deposited in its southern sector adjoining the northern Tianshan Mountains. Two broad types of dolomite are identified in terms of mineralogy, including the homogeneous and anhedral dolomite-A and high order degree and euhedral dolomite-B, which are also largely varied in mineral size (10–20 μm and 30–50 μm in size, respectively). The dolomites display a low degrees of dolomite order (mean = 0.48) and Ca-rich (mean = 55.8). Compared with δ18O values (PDB) of limestones in the shore to shallow facies (−7.0 to −11.1‰), the apparent positive δ18O values of dolomites (−4.2 to −5.9‰) indicate a stronger evaporation environment, while the values of −6.8 to −8.1‰ in the salt marsh facies dolomitic limestone may reflect transitional conditions. Moreover, the more positive δ26Mg values of dolomites (ranging from −1.52 to −2.91‰) than those of limestone and travertine (−3.58‰ and −4.31‰, respectively) also indicate an origin related fractionation effect. In addition, based the occurrence of low-temperature hydrothermal minerals, including chlorite, barite, siderite and silver in the dolostone and travertine, we interpret a new travertine-related origin for the lacustrine dolomite. Thermal water entering saline and alkaline lake environments subject to strong evaporation promoted dolomitization. From the simulation of the Rayleigh and seepage reflux models, we speculate that the lacustrine dolomites formed in a near-surface environment (25–50 °C). Moreover, the results contain abundant information on palaeoenvironmental and palaeoclimatic changes, which are very important for reconstructing and deciphering palaeoenvironments and expanding the dolomitization model under thermal water influenced evaporative lacustrine conditions.