The 200 km 2 Poe Mountain intrusion, part of the 1.43 Ga Laramie anorthosite complex in southeast Wyoming, USA, preserves an exceptional range of primary magmatic and secondary deformation-induced structures that document the sequence of events involved in the emplacement, crystallization, and high-temperature deformation of a Proterozoic anorthosite. The intrusion consists of a steeply dipping (40–90°) olivine leucogabbroic to anorthositic margin of layered cumulates (minimum stratigraphic thickness of 5 km) that contains structures typical of mafic layered intrusions. The layered margin grades inward to a core of granoblastic (recrystallized) anorthosite characteristic of Proterozoic anorthosite plutonic suites worldwide. From the lowest to the highest stratigraphic levels, the layered series is divided into the anorthositic layered zone (ALZ) and leucogabbroic layered zone (LLZ). Igneous layering occurs at all scales from laterally discontinuous cm-thick horizons to distinct mappable packages of layered rocks up to several hundred meters thick. Contacts between the layered zones and internal subsections can be traced ~20 km along strike. The layered rocks are strongly laminated, defined by the orientation of tabular plagioclase; the abundance of layers in the ALZ containing blocky megacrysts of plagioclase increases down-section toward the core. Most of the tabular plagioclase appears to have grown within the upper crustal magma reservoir; plagioclase megacrysts were likely entrained from depth. Isolated examples of layer disruption record periodic re-organization of the semi-consolidated cumulate pile in response to slumping. Structures related to the impact of settled blocks, combined with scours and a mixed magma horizon, indicate that layering and lamination formed directly at or near the magma – pile interface, and that a dynamic magma chamber was present throughout the crystallization history of the Poe Mountain intrusion. Because the rejected interstitial liquid in anorthositic cumulates is denser than intermediate-composition plagioclase, the intrusion floor must have been inclined to allow for drainage. A sloping floor formed in the Poe Mountain intrusion owing to the contemporaneous diapiric rise of the relatively buoyant plagioclase-megacryst-rich rocks in the core; diapirism was the driving force for pervasive recrystallization of the core anorthosite and much of the ALZ. The Poe Mountain intrusion demonstrates that the assembly of Proterozoic anorthosites required active, periodically replenished magma chambers where the efficient segregation of plagioclase and removal of dense rejected melt, aided by syn- to post-crystallization deformation, led to the formation of vast expanses of nearly monomineralic cumulates.
The age and inferred tectonic setting of the 1.76 Ga Horse Creek anorthosite complex (HCAC) in the Laramie Mountains of southeastern Wyoming place important constraints on the origin of middle Proterozoic anorthosite complexes. The 100 km2 HCAC consists of strongly recrystallized anorthosite and two small intrusions of monzonite and granite. U‐Pb crystallization ages from euhedral zircons in anorthosite and monzonite are 1761.5 ± 2 Ma and 1754.5 ± 2.2 Ma, respectively. An additional period of zircon growth in the anorthosite occurred at 1753.4 ± 2 Ma, as represented by a small population of anhedral zircon. We attribute the origin of this second morphological variety of zircon in the anorthosite to the loss of Zr from ilmenite during reaction with plagioclase to form sphene. This reaction took place in response to heat and fluid influx during intrusion of the adjacent monzonite. The HCAC and the younger 1.43 Ga Laramie anorthosite complex to the north were intruded along or near a Paleoproterozoic suture zone, the Cheyenne belt, marking the boundary between Archean rocks of the Wyoming Province to the north and Proterozoic island arc terranes to the south. We propose that the the HCAC was emplaced into young crust during or several million years after collision along the suture in an environment of late‐to post‐orogenic transtension. The presence of pre‐existing crustal structures, especially Archean/Proterozoic boundaries, strongly influences the generation and emplacement of many middle Proterozoic anorthosite complexes.
The Chugwater Anorthosite is one of several 1.43 Ga intrusions that make up the Laramie Anorthosite Complex in the Laramie Mountains, Wyoming, USA. The southwestern portion of the Chugwater Anorthosite has a mappable magmatic stratigraphy totaling at least 8000 meters, and probably 10,000 m or more. It consists of three major units, each of which comprises a lower, dominantly anorthosite portion (>90 vol.% plagioclase) and an upper, dominantly gabbroic anorthosite section (80–90 vol.% plagioclase). Each anorthosite portion contains layers of gabbroic anorthosite and vice versa , on a variety of scales ranging down to centimeters. Plagioclase is dominantly An 50–55 , although higher and lower values are also found. This “main series” of the Chugwater Anorthosite mainly lacks modal and normative olivine. Oxygen fugacity ranged from FMQ to FMQ + 0.5. We interpret that the “main series” was produced by at least three major injections of mildly hyperfeldspathic magma containing approximately 40% entrained plagioclase megacrysts (tabular crystals at least 5 cm across). The Ti contents of the megacrysts (0.3–0.4 wt% TiO 2 ) suggest that these had crystallized at pressures near 10 kbar, consistent with initial formation in a magma chamber at or near the base of the crust. Emplacement pressure is poorly constrained, but appears to have been near 3.5–4 kbar. However, before the main series had completely solidified, it was repeatedly intruded by at least two (and probably more) leucotroctolitic magmas. Contacts of leucotroctolite are sharp against anorthosite, but commonly are diffuse against gabbroic anorthosite, suggesting that sufficient residual melt remained in the latter to permit local mixing with leucotroctolite. We estimate that 80–85% of the Chugwater Anorthosite is main-series, 10–15% is mixed rock, and 5% is leucotroctolite. Prior to final solidification, the entire Chugwater Anorthosite was domed, probably as a result of gravitational instability of the relatively buoyant plagioclase-rich material. The emplacement of the Chugwater Anorthosite as a series of crystal-rich magmas, followed by continued fractionation in a magma chamber at a mid-crustal level and subsequent doming, are characteristics that make the Chugwater Anorthosite intermediate between the Poe Mountain Anorthosite to the north ( in situ fractionation in a magma chamber) and the classic diapiric emplacement of a crystal-rich mush.
Olivine- and pyroxene-bearing Fe-enriched dioritic rocks in the 1434 Ma Laramie anorthosite complex are interpreted to represent variably fractionated and contaminated magmas residual after the crystallization of anorthosite. Geochemical characteristics of this suite include the following: high contents of TiO2, Fe2O3T, and P2O5; high incompatible trace element contents; rare earth element patterns with a large range of Eu anomalies; and isotopic compositions that reflect the geographic location of individual samples, with ISr increasing and ∈Nd decreasing from south to north. After extraction from anorthosite, fractionation of ferrodioritic residual magmas resulted in secondary residual monzodioritic melts and complementary oxide-rich ferrodiorite cumulates. Geographic trends in isotopic composition reflect an increasing Archean crustal component from south to north. Dioritic dikes and cumulates with isotopic compositions similar to associated anorthosites were derived locally. Large isotopic discrepancies between some diorites and their hosting anorthosites reflect preferential contamination of residual magma during ascent and emplacement of mantle-derived plagioclase-rich diapirs, followed by subsequent extraction and isolation of Fe-enriched interstitial melt. Strong isotopic contrasts between anorthosite and associated Fe-enriched rocks in anorthosite complexes do not preclude a direct relationship between them and reflect the diversity and complexity of processes during their petrogenesis.
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