The Medicine Hat Block and the Early Paleoproterozoic Assembly of Western Laurentia

The accretion of the Wyoming, Hearne, and Superior Provinces to form the Archean core of western Laurentia occurred rapidly in the Paleoproterozoic. Missing from Hoffman’s (1988) original rapid aggregation model was the Medicine Hat block (MHB). The MHB is a structurally distinct, complex block of Precambrian crystalline crust located between the Archean Wyoming Craton and the Archean Hearne Province and overlain by an extensive Phanerozoic cover. It is distinguished on the basis of geophysical evidence and limited geochemical data from crustal xenoliths and drill core. New U-Pb ages and Lu-Hf data from zircons reveal protolith crystallization ages from 2.50 to 3.28 Ga, magmatism/metamorphism at 1.76 to 1.81 Ga, and εHfT values from −23.3 to 8.5 in the Archean and Proterozoic rocks of the MHB. These data suggest that the MHB played a pivotal role in the complex assembly of western Laurentia in the Paleoproterozoic as a conjugate or extension to the Montana Metasedimentary Terrane (MMT) of the northwestern Wyoming Province. This MMT–MHB connection likely existed in the Mesoarchean, but it was broken sometime during the earliest Paleoproterozoic with the formation and closure of a small ocean basin. Closure of the ocean led to formation of the Little Belt arc along the southern margin of the MHB beginning at approximately 1.9 Ga. The MHB and MMT re-joined at this time as they amalgamated into the supercontinent Laurentia during the Great Falls orogeny (1.7–1.9 Ga), which formed the Great Falls tectonic zone (GFTZ). The GFTZ developed in the same timeframe as the better-known Trans-Hudson orogen to the east that marks the merger of the Wyoming, Hearne, and Superior Provinces, which along with the MHB, formed the Archean core of western Laurentia.

Download Full-text

40Ar/39Ar evidence for ca. 1800 Ma tectonothermal activity along the Great Falls tectonic zone, central Montana

Canadian Journal of Earth Sciences ◽

10.1139/e02-069 ◽

2002 ◽

Vol 39 (12) ◽

pp. 1719-1728 ◽

Cited By ~ 12

Author(s):

Daniel Holm ◽

David Schneider

Keyword(s):

Tectonic Activity ◽

Geochemical Data ◽

Thermal Activity ◽

Data Set ◽

Tectonic Zone ◽

Rapid Cooling ◽

Magmatic Complex ◽

Wyoming Province ◽

Central Montana

Late Paleoproterozoic (19001600 Ma) tectonothermal activity on all borders of the Archean Wyoming Province has long been established by low-precision KAr and RbSr studies. However, recent tectonic models advanced for supercontinent aggregation require improved constraints on the timing of tectonothermal activity along major boundaries. On its northwestern boundary, the Great Falls tectonic zone separates the Archean Wyoming and Hearne provinces. Recently published U/Pb ages and geochemical data reveal the presence of a Paleoproterozoic (ca. 1860 Ma) marginal-arc magmatic complex along a portion of the Great Falls tectonic zone in central Montana. We present nine new 40Ar/39Ar mineral ages (on hornblende and biotite) from these same arc rocks, which indicate ca. 1800 Ma thermal activity (>500°C) and subsequent rapid cooling to below 300°C by 1775 Ma. This new data set constrains the timing of WyomingHearne collision to between 1860 and 1800 Ma and the timing of last significant tectonothermal activity of this portion of the Great Falls tectonic zone (18001775 Ma). We note that our data add to a growing geochronologic database indicating ca. 1800 Ma tectonothermal activity (via either initial suturing or continued tectonic activity) associated with Paleoproterozoic docking of the Wyoming Province with Laurentia.

Download Full-text

Minette bodies and cognate mica-clinopyroxenite xenoliths from the Milk River area, southern Alberta: records of a complex history of the northernmost part of the Archean Wyoming craton

Canadian Journal of Earth Sciences ◽

10.1139/e00-058 ◽

2000 ◽

Vol 37 (11) ◽

pp. 1629-1650 ◽

Cited By ~ 36

Author(s):

Arndt L Buhlmann ◽

Patricia Cavell ◽

Ronald A Burwash ◽

Robert A Creaser ◽

Robert W Luth

Keyword(s):

Igneous Rocks ◽

Coarse Grained ◽

Geochemical Data ◽

Tectonic Zone ◽

Igneous Complex ◽

Wyoming Craton ◽

Mantle Enrichment ◽

Southern Alberta ◽

Igneous Province ◽

History Of

Minettes exposed in southern Alberta near the Milk River are the northern outliers of the Eocene Sweet Grass Hills igneous complex of the Montana alkalic igneous province. These minettes often contain coarse-grained xenoliths of phlogopite + clinopyroxene ± apatite. The parent magmas of the minettes were generated at pressures [Formula: see text]17 kbar in equilibrium with clinopyroxene + phlogopite ± olivine. Fractional crystallization and mixing provided a spectrum of evolved minettes and cumulates, the latter of which were sampled by subsequent minette magmas as xenoliths. Two xenoliths were dated at 49.0 ± 0.8 Ma and 52 ± 1.7 Ma. The host dyke of the latter xenolith gave an age of 50 ± 0.3 Ma. The minettes and their xenoliths have overlapping values of 87Sr/86Sri, εNdT, 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb, similar to those of alkaline igneous rocks from farther south in the Montana alkalic igneous province. The Sweet Grass Hills lie north of the Great Falls Tectonic Zone, previously interpreted as a Proterozoic suture zone separating the Archean Medicine Hat block from the Archean Wyoming craton to the south. Geochemical data for the Milk River minettes provide evidence for a history of the mantle underneath the Medicine Hat block, similar to that found previously for mantle-derived rocks of the Wyoming craton, including a significant Proterozoic mantle enrichment event. Given this similarity, we suggest that the Wyoming craton extends into southern Alberta, and that the Great Falls Tectonic Zone does not represent a Proterozoic suture of two Archean blocks.

Download Full-text

Lithospheric roots beneath western Laurentia: the geochemical signal in mantle garnets

Canadian Journal of Earth Sciences ◽

10.1139/e03-003 ◽

2003 ◽

Vol 40 (8) ◽

pp. 1027-1051 ◽

Cited By ~ 28

Author(s):

D Canil ◽

D J Schulze ◽

D Hall ◽

B C Hearn Jr. ◽

S M Milliken

Keyword(s):

Rare Earth ◽

North America ◽

Trace Element ◽

Earth Element ◽

Geochemical Data ◽

Trace Element Data ◽

Mantle Lithosphere ◽

Western North America ◽

Wyoming Province ◽

Thick Mantle

This study presents major and trace element data for 243 mantle garnet xenocrysts from six kimberlites in parts of western North America. The geochemical data for the garnet xenocrysts are used to infer the composition, thickness, and tectonothermal affinity of the mantle lithosphere beneath western Laurentia at the time of kimberlite eruption. The garnets record temperatures between 800 and 1450°C using Ni-in-garnet thermometry and represent mainly lherzolitic mantle lithosphere sampled over an interval from about 110260 km depth. Garnets with sinuous rare-earth element patterns, high Sr, and high Sc/V occur mainly at shallow depths and occur almost exclusively in kimberlites interpreted to have sampled Archean mantle lithosphere beneath the Wyoming Province in Laurentia, and are notably absent in garnets from kimberlites erupting through the Proterozoic Yavapai Mazatzal and Trans-Hudson provinces. The similarities in depths of equilibration, but differing geochemical patterns in garnets from the Cross kimberlite (southeastern British Columbia) compared to kimberlites in the Wyoming Province argue for post-Archean replacement and (or) modification of mantle beneath the Archean Hearne Province. Convective removal of mantle lithosphere beneath the Archean Hearne Province in a "tectonic vise" during the Proterozoic terminal collisions that formed Laurentia either did not occur, or was followed by replacement of thick mantle lithosphere that was sampled by kimberlite in the Triassic, and is still observed there seismically today.

Download Full-text

Characterization of crustal xenoliths from the Bearpaw Mountains, Montana (USA), using U-Pb geochronology, whole-rock geochemistry and thermobarometry, with implications for lower crustal processes and evolution of the Wyoming Craton

Chemical Geology ◽

10.1016/j.chemgeo.2019.05.034 ◽

2019 ◽

Vol 524 ◽

pp. 295-322 ◽

Cited By ~ 3

Author(s):

Yashirvad Thakurdin ◽

Robert Bolhar ◽

Péter Horváth ◽

Alexander Rocholl ◽

Kenneth Collerson

Keyword(s):

Wyoming Craton ◽

Crustal Xenoliths ◽

Lower Crustal

Download Full-text

Neotethyan evolution of eastern Greece (Pagondas Mélange, Evia island) inferred from radiolarian biostratigraphy and the geochemistry of associated extrusive rocks

Geological Magazine ◽

10.1017/s0016756801005337 ◽

2001 ◽

Vol 138 (3) ◽

pp. 345-363 ◽

Cited By ~ 36

Author(s):

TANIEL DANELIAN ◽

ALASTAIR H. F. ROBERTSON

Keyword(s):

Shallow Water ◽

Carbonate Platform ◽

Ocean Basin ◽

Passive Margin ◽

Late Triassic ◽

Geochemical Data ◽

Chemical Compositions ◽

Similar Chemical ◽

Thrust Sheets ◽

Age Range

This paper presents new radiolarian biostratigraphic and igneous/metamorphic geochemical data for a Mesozoic volcanic–sedimentary mélange on the island of Evia (Euboea or Evvoia), eastern Greece. This mélange includes dismembered thrust sheets and blocks of radiolarian chert and basalt. Biostratigraphic age data show that radiolarites interbedded with basalt-derived, coarse clastic sediments near the base of a coherent succession were deposited in Middle and Late Triassic time (Late Ladinian–Carnian, Norian?). Geochemical evidence shows that associated extrusive rocks, of inferred Triassic age, range from ‘enriched’ alkaline basalts, to ‘transitional’ basalts, and more ‘depleted’ mid-ocean ridge-type basalts. Amphibolite facies meta-basalts from the metamorphic sole of the over-riding Evia ophiolite exhibit similar chemical compositions. Both the basalts and the meta-basalts commonly show an apparent subduction-related influence (e.g. relative Nb depletion) that may have been inherited from a previous subduction event in the region. The basalts are interpreted to have erupted during Middle–Late Triassic time (Late Ladinian–Carnian), related to initial opening of a Neotethyan ocean basin adjacent to a rifted continental margin. Radiolarites located stratigraphically higher in the coherent succession studied are dated as Middle Jurassic (Late Bathonian–Early Callovian). Similar-aged radiolarites are depositionally associated with ophiolitic rocks (including boninites), in some other areas of Greece and Albania. During initial ocean basin closure (Bajocian–Bathonian) the adjacent shallow-water carbonate platform (Pelagonian zone) disintegrated to form basins in which siliceous sediments were deposited and highs on which shallow-water carbonates continued to accumulate. This facies differentiation is seen as a response to crustal flexure as the Neotethyan ocean began to close. The over-riding Pagondas Mélange and other similar units in the region are interpreted as accretionary prisms related to subduction of Neotethyan oceanic crust in Middle–Late Jurassic time. These mélanges were emplaced, probably diachronously during Oxfordian–Kimmeridgian time, when the passive margin collapsed, creating a foredeep ahead of advancing thrust sheets of mélange and ophiolites.

Download Full-text

The petrogenesis and tectonic setting of lavas from the Baft Ophiolitic Mélange, southwest of Kerman, Iran

Canadian Journal of Earth Sciences ◽

10.1139/e94-076 ◽

1994 ◽

Vol 31 (5) ◽

pp. 824-834 ◽

Cited By ~ 54

Author(s):

Mohsen Arvin ◽

Paul T. Robinson

Keyword(s):

Late Cretaceous ◽

Tectonic Setting ◽

Tholeiitic Basalt ◽

Ocean Basin ◽

Plate Motion ◽

Geochemical Data ◽

Ophiolite Complex ◽

Ophiolitic Mélange ◽

Lut Block ◽

Mafic Lavas

A Late Cretaceous ophiolite complex in the Baft area, southwest of Kerman, Iran, is characteristic of the Central Iranian Ophiolitic Mélange Belt, which wraps around the Lut Block. Despite the extensive tectonic disruption of the Baft complex, most ophiolitic lithologies are present and many original igneous contacts are preserved. A lack of cumulate gabbros within the sequence suggests that a large and continuous magma chamber did not exist beneath the Baft spreading axis. Geochemical data confirm the presence of two distinct compositional groups in the mafic lavas: (1) tholeiitic basalt and (2) transitional tholeiitic basalt. The tholeiitic lavas are similar to typical mid-ocean-ridge basalt compositions, whereas the transitional tholeiites are similar to intraplate basalts. The available data suggest that the Baft ophiolite complex formed in a small ocean basin, possibly at or near a ridge–transform intersection. Emplacement may have occurred as a result of conversion of the transform fault to a subduction zone during a change in relative plate motion. A ridge–transform setting is compatible with the intraplate character of some of the transitional basalts, which probably represent off-axis (seamount) magmatism, marked by the absence of cumulate gabbros and the presence of a serpentinite mélange cut by basaltic dykes. The ridge–transform model suggests formation of the ophiolite in a narrow ocean basin separating the Sanandaj-Sirjan microcontinent from the Central Iran Block in Late Cretaceous time.

Download Full-text