Midcontinent rift volcanism in the Lake Superior region: Sr, Nd, and Pb isotopic evidence for a mantle plume origin

Journal Article · Tue Jul 10 00:00:00 EDT 1990 · Journal of Geophysical Research; (United States)

DOI:https://doi.org/10.1029/JB095iB07p10851· OSTI ID:5446798

Nicholson, S W ^[1]; Shirey, S B ^[2]

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1. Geological Survey, Reston, VA (USA) Univ. of Minnesota, MN (USA)
2. Carnegie Institution of Washington, DC (USA)

Between 1091 and 1098 Ma, most of a 15- to 20-km thickness of dominantly tholeiitic basalt erupted in the Midcontinent Rift System of the Lake Superior region, North American. The Portage Lake Volcanics in Michigan, which are the youngest MRS flood basalts, fall into distinctly high- and low-TiO{sub 2} types having different liquid lines of descent. Incompatible trace elements in both types of tholeiites are enriched compared to depleted or primitive mantle (La/Yb = 4.3-5.3; Th/Ta = 2.12-2.16; Zr/Y = 4.3-4.4), and both basalt types are isotopically indistinguishable. Sr, Nd, and Pb isotopic compositions of the Portage Lake tholeiites have {sup 87}Sr/{sup 86}Sr{sub i} {approx}0.7038, {epsilon}{sub Nd(1095 Ma)} {approx}0 {plus minus} 2, and {mu}{sub 1} {approx}8.2. Model ages with respect to a depleted mantle source (T{sub DM}) average about 1950-2100 Ma. Portage Lake rhyolits fall into two groups. Type I rhyolites have Nd and Pb isotopic characteristics ({epsilon}{sub Nd(1095 Ma)} {approx}0 to {minus}4.7; {mu}{sub 1} {approx}8.2-7.8) consistent with contamination of tholeiitic rocks by 5-10% Archean crust. The one type II rhyolite analyzed has Nd and Pb isotopic compositions ({epsilon}{sub Nd(1095 Ma)} {approx}{minus}13 to {minus}16; {mu}{sub 1} {approx}7.6-7.7) which are consistent with partial melting of Archean crust. Early Proterozoic crust was not a major contaminant of MRS rocks in the Lake Superior region. Most reported Nd and Pb isotopic compositions of MRS tholeiites from the main stage of volcanism in the Lake Superior region and of the Duluth Complex are comparable to the Nd and Pb isotopic data for Portage lake tholeiites. The isotopic enrichment of the MRS source compared to depleted mantle is striking and must have occurred at least 700 m.y. before 1100 Ma.

• OSTI ID: 5446798
• Journal Information: Journal of Geophysical Research; (United States), Vol. 95:B7; ISSN 0148-0227
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
MICHIGAN
VOLCANIC REGIONS
GEOCHEMISTRY
AGE ESTIMATION
BASALT
CONTINENTAL CRUST
EARTH MANTLE
GEOLOGIC HISTORY
ISOTOPE RATIO
LANTHANUM
LEAD ISOTOPES
NEODYMIUM ISOTOPES
ORIGIN
RIFT ZONES
STRONTIUM 86
STRONTIUM 87
TANTALUM
THORIUM
VOLCANISM
YTTERBIUM
YTTRIUM
ZIRCONIUM
ACTINIDES
ALKALINE EARTH ISOTOPES
BETA DECAY RADIOISOTOPES
CHEMISTRY
DEVELOPED COUNTRIES
EARTH CRUST
ELECTRON CAPTURE RADIOISOTOPES
ELEMENTS
EVEN-EVEN NUCLEI
EVEN-ODD NUCLEI
FEDERAL REGION V
GEOLOGIC STRUCTURES
HOURS LIVING RADIOISOTOPES
IGNEOUS ROCKS
INTERMEDIATE MASS NUCLEI
ISOMERIC TRANSITION ISOTOPES
ISOTOPES
METALS
NORTH AMERICA
NUCLEI
RADIOISOTOPES
RARE EARTHS
ROCKS
STABLE ISOTOPES
STRONTIUM ISOTOPES
TRANSITION ELEMENTS
USA
VOLCANIC ROCKS
580000* - Geosciences