Regional Variations in the Lead Isotopic Characteristics of Late Cenozoic Basalts from the Southwestern United States
<p>Over a hundred new analyses on late Cenozoic basalts characterize and delimit regional patterns of lead isotopic behavior in the southwestern United States. The lead isotopic systematics of these volcanic rocks can be divided into three broad regional groups: one characteristic of Colorado...
| Summary: | <p>Over a hundred new analyses on late Cenozoic basalts characterize and delimit regional patterns of lead isotopic behavior in the southwestern United States. The lead isotopic systematics of these volcanic rocks can be divided into three broad regional groups: one characteristic of Colorado Plateau (CP) volcanic fields, a second found in many areas of the Basin and Range (BR) province, and in neighboring areas along the Pacific coast, and a third peculiar to an area in southern Nevada (SN) Colorado Plateau volcanic rocks have generally unradiogenic leads (²⁰⁶Pb/²⁰⁴Pb ≤ 18.7), locally exhibit large variations in isotopic ratio, and define linear arrays (secondary isochrons) on a ²⁰⁶Pb/²⁰⁴Pb-²⁰⁷Pb/²⁰⁴Pb diagram. Basin and Range type samples are characterized by relatively radiogenic leads (18.7 ≤ ²⁰⁶Pb/²⁰⁴Pb ≤ 19.6) and, comparatively, a uniformity of isotopic composition in local areas. Southern Nevada leads are somewhat unradiogenic (18.2 ≤ ²⁰⁶Pb/²⁰⁴Pb ≤ 18.5), appear to display a local uniformity in isotopic composition, and lie off the Colorado Plateau secondary isochrons. Literature sources indicate that southern Nevada basalts are also characterized by relatively high ⁸⁷Sr/⁸⁶Sr ratios (.7060-.7080).</p>
<p>The isotopic properties of volcanic rocks from each of these regions do not appear to record crustal contamination. The general uniformity of isotopic characteristics over large areas (hundreds to thousands of kilometers in size) of the Southwest suggests a fundamental consistency in volcanic source region character over comparable dimensions. Isotopic variations within each province seem to record smaller scale source region heterogeneities.</p>
<p>Isotopic variability observed within individual Colorado Plateau volcanic fields may be interpreted in terms of kilometer-sized mantle heterogeneities developed approximately 1.4-1.8 by ago. These heterogeneities may record the effects of magma extraction from mantle sources during the primary generation of Precambrian crystalline basement and the formation of the continental lithosphere. A rough but apparently significant correlation between the chemical and lead isotopic compositions of CP basalts suggests that the mantle of the continental lithosphere may be crudely "stratified" in its lead isotopic composition, with the uppermost mantle generally less radiogenic in lead (and by inference more depleted in uranium relative to lead) than the deeper lithosphere. Isotopic differences distinguishable between different portions of the Colorado Plateau may relate to heterogeneities hundreds of kilometers in size which developed in different mantle domains prior to and in the course of continental lithosphere formation.</p>
<p>Basin and Range basalts have lead isotopic properties (and Sr and Nd isotopic compositions) similar to volcanic rocks from oceanic island and volcanic arc settings, suggesting a correspondence between BR volcanic source regions and "oceanic" mantle. Within the Basin and Range province ²⁰⁶Pb/²⁰⁴Pb ratios tend to be rather uniform laterally for distances measured in hundreds of kilometers; this implies that the underlying mantle may be isotopically rather homogeneous for similar distances. One 200 km long section of the Rio Grande rift is characterized by CP type leads. This may possibly indicate that a large body of "oceanic" mantle has penetrated the continental lithosphere beneath this segment of the Rio Grande rift.</p>
<p>Older (Pliocene and Miocene) calc-alkaline volcanic rocks from the Basin and Range province tend to display somewhat higher ²⁰⁷Pb/²⁰⁴Pb ratios than the Quaternary alkaline basalts within each area of the Basin and Range province. This could suggest that adjustments in BR type source region characteristics continued very late into the Cenozoic era, perhaps in conjunction with changes in the tectonic environment.</p>
<p>SN type isotopic systematics resist a unique interpretation, but may reflect long term (of the order of a billion years or so) evolutionary characteristics of a source region in the mantle or the lower crust.</p>
<p>Although the different isotopic province boundaries appear to record discontinuities deep within the earth (most probably fundamental discontinuities in mantle character), they follow the trends of major geological and structural boundaries in the crust. The SN isotopic province, for example, appears to be associated geographically with a "fundamental, west-trending, transverse crustal boundary" (Eaton, 1975) which demarcates the northern and southern portions of the Basin and Range province. The Basin and Range - Colorado Plateau isotopic province boundary approximately follows the Mesozoic-early Tertiary Cordilleran foreland thrust belt and the limit of late Cenozoic normal faulting. Thus the BR-CP isotopic boundary appears to be geographically related to the boundary between tectonically unstable, "orogenic" areas of the Basin and Range province and Pacific border regions, and the more stable "platform" areas of the Colorado Plateau, Southern Rocky Mountains, and Great Plains.</p>
<p>BR type leads occur in some areas (notably southern Arizona and southeastern California) characterized by known Precambrian basement. These areas were presumably underlain originally by "old" mantle belonging to the continental lithosphere. The inferred presence of BR type, "oceanic" mantle beneath these regions during late Cenozoic times seems to imply a disruption of continental lithosphere and the emplacement of "new" oceanic mantle. The geographic association of Mesozoic and Cenozoic structural boundaries in the crust and the BR-CP isotopic boundary suggests that this mantle disruption may be related either to the compressional tectonism of Mesozoic-early Tertiary times or to the extensional tectonism of the late Cenozoic.</p> |
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