Abstract
Olistolith production and magmatism are processes commonly associated with extensional tectonic settings, such as rift basins. We present a cautionary exemplar from one such Precambrian basin, in which we reinterpret metabasite bodies, previously documented as sills, to be olistoliths. We nevertheless demonstrate that, on the basis of field observation alone, the previous but erroneous sill interpretation is parsimonious. Indeed, it is only by using isotopic age and compositional analysis that the true identities of these metabasite olistoliths are revealed. We present new data from metabasites and metasedimentary strata of the Kingston Peak Formation (Cryogenian) and Crystal Spring Formation (Mesoproterozoic) of Death Valley, USA. These include field observations, U–Pb apatite ages, U–Pb zircon ages (detrital and igneous) and whole-rock geochemistry. These data also provide a new maximum age for the base of the Pahrump Group and suggest that the Crystal Spring Diabase was more tholeiitic than previously thought. Similar sill/olistolith misinterpretations may have occurred elsewhere, potentially producing erroneous age and tectonic-setting interpretations of surrounding strata. This is particularly relevant in Precambrian rocks, where fossil age constraints are rare. This is illustrated herein using a potential example from the Neoproterozoic literature of the Lufilian belt, Africa. We caution others against Precambrian olistoliths masquerading as sills. Supplementary material: Details of a meta-igneous boulder from P12 of the Silurian Hills, LA-ICP-MS and whole-rock geochemistry methods and standards, and U–Pb apatite and zircon isotopic data, including standards and selected cathodoluminescence images, are available at https://doi.org/10.6084/m9.figshare.c.3990639
Highlights
Isotopic age and geochemical data suggest that the Silurian Hills Kingston Peak undifferentiated (KPu) metabasites have ages of around 1.1 Ga and are genetically related to the Crystal Spring Diabase (CSD)
Detrital zircon data of this study place the Crystal Spring unconformity hundreds of metres below the KPu in the Silurian Hills. This confirms the existing consensus (e.g. Basse 1978; Maud 1979; Shafer 1983; Smith et al 2016) and renders a Mesoproterozoic glaciation unlikely. The latter hypothesis is well precedented in the southern Kingston Range, where field observation alone allowed an olistolith interpretation (Calzia et al 2000)
In the Silurian Hills, reliance on field observation alone has hitherto allowed the sill interpretation to remain unchallenged. This is despite carbonate olistoliths, in the KPu of the Silurian Hills, alerting us to the possibility of metabasite olistoliths (Fig. 2a)
Summary
To investigate the stratigraphic context, age and source of the metabasite bodies in the KPu of the Silurian Hills, we present the following. (1) Whole-rock geochemistry, field observations, U–Pb apatite and igneous zircon ages from KPu metabasite bodies of the Silurian Hills (samples SH1 to -5) (Table 1). (3) Whole-rock geochemistry and field observations of a pillow lava (sample HC1) from the KPu of the Panamint Range (Table 1), as a comparison with the Silurian Hills KPu metabasite bodies. DZ-SH3, yielded an age spectrum very similar to those from above the Crystal Spring unconformity throughout the region, including the Silurian Hills samples of Smith et al (2016) (Fig. 12) These data suggest that a distinct Crystal Spring Formation detrital zircon age signature may be recognized in separate localities around the region and used as a correlation tool. It should be noted that the three youngest grains from sample K03DV04 (c. 1320 Ma; Mahon et al 2014a) (Fig. 12) are from a different and younger depositional system (Roberts 1974, 1976; Mahon et al 2014a) and do not reflect a maximum age for the Pahrump Group (see Mulder et al 2017)
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