Epidosites (epidote- and quartz-rich rock with granoblastic texture) from the Solea graben are believed to form from upflowing hydrothermal fluid that deposited massive sulfide ores at the seawater-sea-floor interface of the Cretaceous age oceanic crust. Epidosite within the graben occurs as massive epidosite (epidosite concentration greater than 80%) and zones of incipient epidotization (between 5% and 80%). Regions of incipient epidotizations are characterized by the juxtaposition of greenschist facies alteration and epidosite; the fluids responsible for each of these alteration styles had similar oxygen isotope compositions and temperatures. Petrographic observations suggest that the epidosite protolith was a previously altered rock....
Recent studies demonstrate that rifts are characterized by linked tilt domains, each containing a consistent polarity of normal faults and stratal tilt directions, and that the transition between domains is typically through formation of accommodation zones and generally not through production of throughgoing transfer faults. The mid‐Miocene Black Mountains accommodation zone of southern Nevada and western Arizona is a well‐exposed example of an accommodation zone linking two regionally extensive and opposing tilt domains. In the southeastern part of this zone near Kingman, Arizona, east dipping normal faults of the Whipple tilt domain and west dipping normal faults of the Lake Mead domain coalesce across a relatively narrow region characterized by a series of linked, extensional folds. The geometry of these folds in this strike‐parallel portion of the accommodation zone is dictated by the geometry of the interdigitating normal faults of opposed polarity. Synclines formed where normal faults of opposite polarity face away from each other whereas anticlines formed where the opposed normal faults face each other. Opposed normal faults with small overlaps produced short folds with axial trends at significant angles to regional strike directions, whereas large fault overlaps produce elongate folds parallel to faults. Analysis of faults shows that the folds are purely extensional and result from east/northeast stretching and fault‐related tilting. The structural geometry of this portion of the accommodation zone mirrors that of the Black Mountains accommodation zone more regionally, with both transverse and strike‐parallel antithetic segments. Normal faults of both tilt domains lose displacement and terminate within the accommodation zone northwest of Kingman, Arizona. However, isotopic dating of growth sequences and crosscutting relationships show that the initiation of the two fault systems in this area was not entirely synchronous and that west dipping faults of the Lake Mead domain began to form between 1 m.y. to 0.2 m.y. prior to east dipping faults of the Whipple domain. The accommodation zone formed above an active and evolving magmatic center that, prior to rifting, produced intermediate‐composition volcanic rocks and that, during rifting, produced voluminous rhyolite and basalt magmas.
Abstract Five basalt samples from the Point Sal ophiolite, California, were examined using HRTEM and AEM in order to compare observations with interpretations of XRD patterns and microprobe analyses. XRD data from ethylene‐glycol‐saturated samples indicate the following percentages of chlorite in mixed‐layer chlorite–smectite identified for each specimen: (i) L2036 ± 50%, (ii) L2035 ± 70 and 20%, (iii) 1A‐13 ± 70%, (iv) 1B‐42 ± 70%, and (v) 1B‐55 = 100%. Detailed electron microprobe analyses show that ‘chlorite’analyses with high Si, K, Na and Ca contents are the result of interlayering with smectite‐like layers. The Fe/(Fe + Mg) ratios of mixed‐layer phyllosilicates from Point Sal samples are influenced by the bulk rock composition, not by the percentage of chlorite nor the structure of the phyllosilicate. Measurements of lattice‐fringe images indicate that both smectite and chlorite layers are present in the Point Sal samples in abundances similar to those predicted with XRD techniques and that regular alternation of chlorite and smectite occurs at the unit‐cell scale. Both 10‐ and 14‐Å layers were recorded with HRTEM and interpreted to be smectite and chlorite, respectively. Regular alternation of chlorite and smectite (24‐Å periodicity) occurs in upper lava samples L2036 and 1A‐13, and lower lava sample 1B‐42 for as many as seven alternations per crystallite with local layer mistakes. Sample L2035 shows disordered alternation of chlorite and smectite, with juxtaposition of smectite‐like layers, suggesting that randomly interlayered chlorite (<0.5)–smectite exists. Images of lower lava sample 1B‐55 show predominantly 14‐Å layers. Units of 24 Å tend to cluster in what may otherwise appear to be disordered mixtures, suggesting the existence of a corrensite end‐member having thermodynamic significance.
