Climate controlled, fabric destructive, reflux dolomitization and stabilization via marine- and synorogenic mixed fluids: An example from a large Mesozoic, calcite-sea platform, Croatia

Abstract

The Jurassic–Cretaceous dolomites, Adriatic platform, Croatia resulted from climate-influenced post-depositional reflux dolomitization (as opposed to synsedimentary peritidal- and deeper burial dolomitization), and subsequent stabilization within a Mesozoic, “calcite sea” isolated platform. The dolomites are stratiform (10 to 200m thick), fabric destructive (20 to over 500μm crystals), nonluminescent, and zoned with respect to Ca. Bulk dolomites have low Mn (10 to 30ppm), moderate Sr (60 to over 200ppm), positive δ13C and δ18O values, only moderate ordering (0.25 to 0.6) and single-phase fluid inclusions (temperatures <50°C). High-Ca dolomite (HCD; ~53 to ~59mol% Ca; δ18O+1 to >+3‰ VPDB and Sr>100ppm) composes most Jurassic and 40% of Cretaceous dolomites, making up turbid dolomite cores and initial clear dolomite rims. A first generation of low-Ca dolomite (LCD-1; 50 to 53mol% Ca; δ18O+1 to >+3‰ VPDB; 100–180ppm Sr) forms cement and variably replaces (stabilizes) earlier HCD cores. HCD and LCD-1 formed in refluxing marine-dominated pore waters under semi-arid climate (<300m depth, 30 to ~40°C). 87/86Sr values of the HCD and LCD-1 dominantly were controlled by the refluxing seawater 87/86Sr with minor addition of 87/86Sr from ascending cooling fluids. During maturation of HCD, Sr loss was greatest from least stable, more calcic HCD (>55mol%) phases. A second generation of much younger, low-Ca dolomite (LCD-2; fracture-associated, more negative δ18O from −1.4 to +1‰ VPDB, Sr <100ppm, enriched in radiogenic 87Sr) overgrows and variably replaces earlier Cretaceous dolomites. It formed during Eocene–Oligocene deformation and uplift to <1km burial depths (<50°C) from synorogenic, mixed marine–meteoric pore fluids, circulating via faults, fractures and local permeable zones. Porosity loss by dolomite cementation coupled with localized, rapid Cenozoic uplift, variably arrested stabilization to LCD.