ALBERT

Description: The Andalshatten batholith (322 km 2 , 〉700 km 3 ) is a predominantly granodioritic high-K, calc-alkaline igneous body that was assembled in the mid-crust across four lithologically distinct nappes within the Helgeland Nappe Complex, central Norway. Extensive vertical and horizontal exposures of metamorphic screens and xenoliths within the batholith provide an unparalleled view of the nature of magma emplacement, host rock displacement, and batholith assembly, i.e., batholith tectonics. The mapped intrusion consists of at least five distinct lithologic phases, including schlieren-banded to gneissic granodiorite (11% of batholith area), coarse-crystalline to K-feldspar megacrystic granodiorite (69%), amphibole-bearing diorite (11%), tonalite (2%), and minor leucogranite. Contacts between phases are both sharp and/or gradational and are interpreted to reflect comagmatic behavior over the duration of crystallization of the phases separated by a given contact. New chemical abrasion–thermal ionization mass spectrometry 206 Pb/ 238 U zircon weighted mean ages of 442.67 ± 0.14 Ma and 441.53 ± 0.40 Ma for 2 samples of the voluminous megacrystic granodiorite from disparate localities indicate distinct periods of zircon crystallization separated by ~1 Ma; titanite ages for these samples are 441.30 ± 0.21 Ma and 436.10 ± 2.80 Ma, respectively. No observable contacts were identified between these two lithologically similar localities. Of the mapped intrusion area, ~8% (〉24 km 2 ) comprises screens (kilometer scale) and xenoliths (subkilometer scale) of metamorphic rocks that reflect the skeletal framework of the host rock nappes into which the granodioritic magmas intruded. This ghost stratigraphy maintains broad continuity with host rock lithology and structural trends. The largest screens show no evidence of internal, emplacement-related ductile deformation, but appear to be rigidly rotated into subparallelism with the western host rock contact, presumably during subsequent magma injection into the batholith. In contrast, xenoliths underwent rotation, translation, and internal deformation in the magma. The scale dependence of synmagmatic deformation of screens and xenoliths is likely the result of smaller blocks becoming thermally equilibrated with the surrounding magma and thus deforming by ductile mechanisms in a magma with increasing yield strength due to crystallization. We interpret the Andalshatten batholith to have been assembled by at least five spatially distinct, elongate batches of magma over a minimum duration 600 ka to 1.7 Ma, including significant recharge events involving dioritic magmas. Local space for batholith assembly was accommodated by brittle and ductile deformation, including viscous flow of host rocks in a dynamothermal contact aureole. Viscous flow was facilitated by reactivation of existing structures (e.g., tightening of interlimb fold angles), recrystallization, and penetrative foliation development, resulting in near-field lateral and downward-directed displacement of host rocks along the western margin during batholith expansion and growth. Emplacement of dioritic magmas added heat and mass to the growing reservoir, enabling significant magnitudes of internal, hypersolidus flow, magmatic foliation development, mechanical mingling, and screen deformation. These observations and data sets are consistent with the hypothesis of multiple recharge events in a magma chamber that was partially molten over reasonably large spatial scales, thereby allowing screens and xenoliths to be incorporated and displaced and/or deformed.

Description: Paleoproterozoic suture zones mark the formation of supercontinent Nuna and provide a record of North America’s assembly. Conspicuously young ages (ca. 1.715 Ga) associated with deformation in southeast Wyoming craton argue for a more protracted consolidation of Laurentia, long after peak metamorphism in the Trans-Hudson orogen. Using paleomagnetic data from the newly dated 1899 ± 5 Ma Sourdough mafic dike swarm (Wyoming craton), we compare the relative positions of Wyoming, Superior, and Slave cratons before, during, and after peak metamorphism in the Trans-Hudson orogen. With these constraints, we refine a collisional model for Laurentia that incorporates Wyoming craton after Superior and Slave cratons united, redefining the Paleoproterozoic sutures that bind southern Laurentia.

Description: The Wooley Creek batholith is a tilted, zoned, calc-alkaline plutonic complex in the Klamath Mountains, northern California, USA. It consists of three main compositional-temporal zones. The lower zone consists of gabbro through tonalite. Textural heterogeneities on the scale of tens to hundreds of meters combined with bulk-rock data suggest that it was assembled from numerous magma batches that did not interact extensively with one another despite the lack of sharp contacts and identical ages of two lower zone samples (U-Pb [zircon] chemical abrasion–isotope dilution–thermal ionization mass spectrometry ages of 158.99 ± 0.17 and 159.22 ± 0.10 Ma). The upper zone is slightly younger, with 3 samples yielding ages from 158.25 ± 0.46 to 158.21 ± 0.17 Ma, and is upwardly zoned from tonalite to granite. This zoning can be explained by crystal-liquid separation and is related to upward increases in the proportions of interstitial K-feldspar and quartz. Porphyritic dacitic to rhyodacitic roof dikes have compositions coincident with evolved samples of the upper zone. These data indicate that the upper zone was an eruptible mush that crystallized from a nearly homogeneous parental magma that evolved primarily by upward percolation of interstitial melt. The central zone is a recharge area with variably disrupted synplutonic dikes and swarms of mafic enclaves. Central zone ages (159.01 ± 0.20 to 158.30 ± 0.16 Ma) are similar to both lower and upper zones crystallization ages. In the main part of the Wooley Creek batholith, age data constrain magmatism to a short period of time (〈1.3 m.y.). However, age data cannot be used to identify distinct magma chambers within the batholith; such information must be extracted from a combination of field observations and the chemical compositions of the rocks and their constituent minerals.

Description: The interval between 1.78 and 1.63 Ga was one of major crustal growth and assembly in southwestern Laurentia. The prevailing view is that the culminating event for this tectonism was the ca. 1.654–1.633 Ga Mazatzal orogeny. We present evidence for a continuum of deformation, magmatism, metamorphism, and new mineral growth from 1.65 to 1.58 Ga from southern Wyoming to Sonora, Mexico. We suggest that the Mazatzal orogeny may have extended in time to ca. 1580 Ma and in space to the Cheyenne belt, southern Wyoming. If our interpretation for an extended Mazatzal orogeny is correct, the duration of the orogeny may have been ~70 m.y., similar to many orogenies in Earth’s history. The ca. 1.6 Ga tectonism appears to represent a shift in tectonic style from that typically associated with the Yavapai (ca. 1.70 Ga) and "classic" Mazatzal (1.65 Ga) orogenies to widespread intracontinental deformation. If correct, a corollary to our interpretation is that newly accreted Paleoproterozoic crust stabilized rapidly and facilitated stress transfer far inboard of any active plate margin.

Description: Paleoproterozoic suture zones mark the formation of supercontinent Nuna and provide a record of North America’s assembly. Conspicuously young ages (ca. 1.715 Ga) associated with deformation in southeast Wyoming craton argue for a more protracted consolidation of Laurentia, long after peak metamorphism in the Trans-Hudson orogen. Using paleomagnetic data from the newly dated 1899 ± 5 Ma Sourdough mafic dike swarm (Wyoming craton), we compare the relative positions of Wyoming, Superior, and Slave cratons before, during, and after peak metamorphism in the Trans-Hudson orogen. With these constraints, we refine a collisional model for Laurentia that incorporates Wyoming craton after Superior and Slave cratons united, redefining the Paleoproterozoic sutures that bind southern Laurentia.