Abstract
Chrome chlorites are usually found as secondary phases formed by hydrothermal alteration of chromite deposits and associated mafic/ultramafic rocks. Here, we report the 40Ar/39Ar age of chrome chlorites separated from the Luobusa massive chromitites which have undergone secondary alteration by CO2-rich hydrothermal fluids. The dating results reveal that the intermediate heating steps (from 4 to 10) of sample L7 generate an age plateau of 29.88 ± 0.42 Ma (MSWD = 0.12, plateau 39Ar = 74.6%), and the plateau data points define a concordant inverse isochron age of 30.15 ± 1.05 Ma (MSWD = 0.08, initial 40Ar/36Ar = 295.8 ± 9.7). The Ar release pattern shows no evidence of later degassing or inherited radiogenic component indicated by an atmospheric intercept, thus representing the age of the hydrothermal activity. Based on the agreement of this hydrothermal age with the ~30 Ma adakitic plutons exposed in nearby regions (the Zedong area, tens of kilometers west Luobusa) and the extensive late Oligocene plutonism distributed along the southeastern Gangdese magmatic belt, it is suggested that the hydrothermal fluids are likely related to the ~30 Ma magmatism. The hydrothermal fluid circulation could be launched either by remote plutons (such as the Sangri granodiorite, the nearest ~30 Ma pluton west Luobusa) or by a similar coeval pluton in the local Luobusa area (inferred, not found or reported so far). Our results provide important clues for when the listwanites in Luobusa were formed.
Highlights
Over the past two decades, podiform chromitites in Luobusa, Tibet, and the host peridotites were extensively studied to understand their petrogenesis and related tectonic evolutions
Despite the fact that chromites within chromitites and peridotites are in general better preserved than silicates as they are more resistant to fluid-related process, textural and chemical alteration of chromites may occur during retrograde/prograde metamorphic events and depend on the degrees of metamorphism (e.g., [22,23,24])
In some cases, tiny scattered minerals, with higher reflectivity occurring along the contact between chrome chlorites and chromites within the chromite’s side, are probably ferritchromites (Figure 2e,f)
Summary
Over the past two decades, podiform chromitites in Luobusa, Tibet, and the host peridotites were extensively studied to understand their petrogenesis and related tectonic evolutions. As a further part of this work, Zhang et al [28] recently refined the mineral-transformation processes corresponding to different degrees of alteration intensity during listwanite formation They noted a preliminary enrichment of gold and mercury in the listwanites. The term “chrome chlorite” is used for the violet or pink colored chlorite containing chromium to differentiate it from the green Fe-Mg chlorite [32], the nomenclature of chrome chlorite is complicated based on its Cr2 O3 contents and the position of chromium substituting for Al in the chlorite structure (e.g., [33]) They are usually formed as secondary phases during hydrothermal alteration of podiform chromitites and associated mafic/ultramafic rocks (e.g., [27,32,34,35]). The pinkish hydrothermal chrome chlorites within the massive chromitites in Luobusa are first reported and dated by the Ar/Ar technique to provide age constraints for the secondary hydrothermal event
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