Chlorite, Corrensite, and Chlorite-Mica in Late Jurassic Fluvio-Lacustrine Sediments of the Cameros Basin of Northeastern Spain

Abstract

Abstract--The distribution and crystal-chemical characteristics of chlorite, eorrensite, and mica in samples from a stratigraphic profile in the Cameros basin are controlled by changes in the sedimentary facies. The lacustrine marls and limestones from the base and the top of the profile contain quartz + calcite + illite -+ dolomite +-chlorite -+ albite _+ paragonite + Na, K-rich mica. Chlorite is rich in Mg, with Fe/ (Fe + Mg) ratios ranging between 0.18-0.37. A formation mechanism involving reaction between Mgrich carbonate and dioctahedral phyllosilicates is proposed for these Mg-rich chlorites, on the basis of the mutually exclusive relationship found between Mg-rich chlorite and dolomite, together with the relative increase in the proportion of calcite in samples containing chlorite.The mudrocks from the middle part of the profile are composed of quartz + albite + illite + corrensite (with a mean coefficient of variability of 0.60%) + chlorite. Corrensite and chlorite are richer in Fe 2+ than those from the base or top of the profile, with mean Fe/(Fe + Mg) ratios of 0.51 and 0.56, respectively. Textural and compositional features suggest a formation mechanism for the corrensite, chlorite, and chlorite-mica crystals through replacement of detrital igneous biotite. Whether or not corrensite occurs with chlorite appears to be related to redox conditions. The presence of corrensite alone is apparently favored by oxidizing conditions, whereas the occurrence of corrensite + chlorite is related to more reducing conditions. Corrensite shows higher Si and Na + K + Ca contents, and slightly lower Fe/(Fe + Mg) ratios than chlorite. The presence of corrensite and the lack of random chlorite-smectite interlayering is discussed in terms of the fluid/rock ratio; the occurrence is related to the hydrothermal character of metamorphism in the Cameros basin.

Structural and temporal evolution of a reactivated brittle–ductile fault – Part I: Fault architecture, strain localization mechanisms and deformation history

“…Chlorite, as other phyllosilicates, has a low friction coefficient (μ ∼ 0.3; Behnsen and Faulkner, 2012) compared to the bulk strength of most crustal rocks (0.6-0.85; Byerlee, 1978), and therefore can slide at relatively low differential stresses. However, in the case of the San Andreas Fault it has been argued that the frictional strength of chlorite is still too high to accommodate stable creep by frictional grain sliding alone, and that other mechanisms have to be active (e.g.…”

Micro-Sized Dolomite Inclusions in Ferroan Calcite Cements Developed During Burial Diagenesis of Kimmeridgian Reefs, Northern Iberian Basin, Spain

“…On the basis of a detailed study of the mineralogical, petrological, geochemical, and structural attributes of the basin, this metamorphic episode has been characterized as hydrothermal and allochemical (Casquet et al 1992;Barrenechea et al 1995Barrenechea et al , 2000Barrenechea et al 2001;Alonso-Azcárate et al 1995Alonso-Azcárate et al1999a, Alonso-Azcárate 1999b, Alonso-Azcárate 2001a, Alonso-Azcárate 2001bMantilla-Figueroa et al 1998;Mas et al 2003). Microthermometry of fluid inclusions, radiometric data, mineral assemblages (chloritoidchlorite at peak metamorphism), crystallochemical properties of phyllosilicates, chlorite microthermometry, isotopic thermometry, and geochemistry of pyrite deposits indicate that: (1) metamorphism displays clear thermal inversions across areas of rapid deposition; (2) metamorphic grade is more influenced by the changes in rock permeability and composition than by burial depth; (3) metamorphism postdated the filling of the basin (post-rift metamorphic ages range from 106 to 86 Ma (late Albian-Coniacian); and, (4) the metamorphism ranged from low-grade (epizone) to very low-grade (anchizone) conditions, with maximum temperatures and pressures of 350 to 370uC, and 1 kbar, respectively.…”

“…Upper Jurassic-Lower Cretaceous deposits of the Cameros Basin are composed mainly of Fe-rich sandstones and Fe-and Mg-rich clay minerals. This metamorphic event led to extensive transformation of clays and associated minerals in the northwestern parts of the basin (AlonsoAzcárate et al 1995(AlonsoAzcárate et al , 2005Alonso-Azcárate et al 1999b;Barrenechea et al 2000;Barrenechea et al 2001) (Fig. 1A).…”

Easily altered minerals and reequilibrated fluid inclusions provide extensive records of fluid and thermal history: gypsum pseudomorphs of the Tera Group, Tithonian-Berriasian, Cameros Basin

“…1) [19][20][21][22][23][24][25][26]. The main features of this thermal alteration are: (1) metamorphic grade is controlled by changes in rock composition and permeability rather than by burial depth [20,21,27,28]; (2) thermal inversions across sections in the depocentre [24,25,28]; (3) post-rift age of alteration (107±5 to 85±6 My dated by K-Ar on authigenic illites [19] or 99±2 My by U-Pb SHRIMP on monazites [29]) after the maximum burial stage, reached during the Early Albian; and (4) metamorphic conditions from very low-grade (anchizone) to low-grade (epizone), with temperatures of 350-370…”