Juvenile Proterozoic Crust Research Articles

Magmatism and metallogeny associated with mantle upwelling: Zircon U–Pb and Lu–Hf constraints from the gold-mineralized Jinchang granite, NE China

The Jinchang gold deposit is located in the easternmost portion of the Central Asian Orogenic Belt (CAOB), and represents one of the major gold districts in eastern Jilin–Heilongjiang provinces of China. The gold ore bodies are hosted mainly in altered Mesozoic granitoids, breccia pipes and ring and radial faults. Gold mineralization consists of alteration (stockwork in hydrothermally altered granites), breccia, and quartz-sulfide vein-types. Alteration assemblages around the alteration-style ore body show a vertical sequence of potassic, phyllic, and propylitic zones.In this study, we present U–Pb and Lu–Hf isotope data on zircons derived from mineralized granophyric granite, biotite monzogranite, granodiorite, and granite porphyry, and sericite Rb–Sr ages from the Jinchang gold deposit. The results show 206Pb/238U ages of 201±3Ma (MSWD=1.1), 203±4Ma (MSWD=1.4), 201±5Ma (MSWD=2.1), and 110±3Ma (MSWD=1.6), respectively. Sericite from the gold-mineralized phyllic-altered granodiorite and granite porphyry returns Rb–Sr isochron ages of 110±4Ma (MSWD=1.04) and 107±5Ma (MSWD=0.91), respectively. Our new data indicate that the gold mineralization at Jinchang took place at ca. 110Ma and was temporally related to intrusion of the granite porphyry. Zircon ε Hf (200Ma) values of the ca. 200Ma granites vary from −4.8 to +8.1, with TCDM model ages of 727–1535Ma, reflecting their derivation mainly by partial melting of juvenile Proterozoic crust. The gold-bearing 110Ma granite porphyry returns ε Hf (110Ma) values in the range of −1.6 to +9.8, with TCDM model ages of 542–1069Ma, suggesting partial melts of juvenile Proterozoic crust with notable input of mantle components as compared to the ca. 200Ma granites. Compiled oxygen (δ18OSMOW=−0.7–10.1) and hydrogen (δDSMOW=−99 to −70) stable isotopic values of quartz from ores indicate that the ore-forming fluids were predominantly exsolved from magmas with minor amount of meteoric water in quartz-sulfide veins at the late stage. The Hf isotope data from the granite porphyry, integrated with the results from previous data on S and Pb isotopic composition of ores, constrain the source of ore-forming components as lower crust with discernible mantle inputs and wall rock assimilation. Our results have implications bearing on the widespread magmatism and metallogenic event during the Early Cretaceous time in East China, and link them to mantle upwelling that contributed both heat and volatiles for crustal melting and scavenging of metals which in turn were concentrated in upper crustal levels through exsolution for the magmas.

Hafnium isotopes in zircon: A tracer of fluid-rock interaction during magnetite–apatite (“Kiruna-type”) mineralization

In situ analyses of hafnium isotopes in zircon combined with U–Pb zircon ages, and trace element data, provide new information on the characterization and evolution of magnetite–apatite (“Kiruna-type”) deposits and the tectonic environments in which they occur. The Lyon Mountain granite in the Adirondack Mountains of New York is the host to numerous zircon-bearing magnetite–apatite deposits. Hafnium isotopic compositions and rare earth element contents in individual zircon crystals were measured in situ in both the host granites and the ore bodies by laser ablation inductively coupled plasma-mass spectrometry. Hafnium isotopic compositions in the ore zircon can be divided into two groups: those that have initial εHf (t) values that are indistinguishable from those of the host granites (e.g., εHf (t) less than +7) and are typical of relatively juvenile Proterozoic crust, and those that have extremely radiogenic εHf (t) values (as high +40). Two models are proposed to explain the observed εHf (t) values in the zircon crystals: 1) early-formed ore bodies containing magnetite, apatite, and clinopyroxene were remobilized by secondary fluid alteration, releasing Zr and Hf for the crystallization of new zircon; or alternatively, 2) fluids responsible for ore formation have interacted with garnet-bearing rocks during retrograde metamorphism, scavenging rare earth elements and radiogenic Hf.Previous work done to determine the U–Pb ages from the same zircon crystals, which were analyzed for the Hf isotopic composition in this study, revealed that ore bodies record a mineralizing event that is 20 to 60m.y. younger than the age of granite emplacement. This age discrepancy, plus the highly radiogenic εHf (t) values in the ore zircon crystals, suggests that the fluids responsible for this younger event could not have been derived from the granite hosts. These data argue that magnetite–apatite deposits in the LMG have multiple mineralizing events superimposed upon one another and that early-formed deposits may be reworked, modified and redeposited by fluids subsequent to magma crystallization.

Heat flow in the Trans‐Hudson Orogen of the Canadian Shield: Implications for Proterozoic continental growth

Fourteen new heat flow and radiogenic heat production measurements have been obtained in the Paleo‐Proterozoic Trans‐Hudson Orogen of the Canadian Shield. This orogen, which consists of several distinctive belts, corresponds to a pulse of crustal growth through island arc magmatism between 1.9 and 1.8 Ga. The data now available include 17 previously published measurements. Heat flow variations that are related to the history of magmatism and internal differentiation of the belts provide constraints on the crustal assemblages in the different belts of the orogen. The average and standard deviation of heat flow values for the entire orogen, 42±11 mW m−2, are identical to those of the older Superior Province and of the younger Grenville Province. For the orogen as a whole, heat flow is weakly correlated to the heat production of surface rocks. High heat flow values are found in the Thompson belt, consisting of metasedimentary rocks deposited on the ancient continental margin of the Superior craton. There the accumulation of sediments derived from older and differentiated continental upper crust has resulted in significant concentrations of radioelements in large volumes of rocks. The heat flow is low in the belts that expose juvenile Proterozoic crust consisting mostly of arc‐related volcanic rocks. In the Flin Flon‐Snow Lake Belt, the average heat flow is the same as the average of the orogen. The low heat production and the lack of correlation between heat flow and heat production suggest that the supracrustal volcanics exposed at the surface are thin and rest on a basement richer in radioelements. In the Lynn Lake belt, the heat flow is significantly lower than the average for the orogen although the surface heat production is not low. The heat flow data require a thin (<10 km) surface layer overlying the mid and lower crust depleted in radioelements. Around the town of Lynn Lake, heat flow is consistently low over a distance of ≈40 km. The coincidence between this “cold spot” and anomalously thick crust suggests that deep crustal roots may be preserved because of the stronger rheology implied by the low temperatures. The evolution of the Trans‐Hudson Orogen exemplifies the interplay between the processes generating rocks of evolved composition, which require crustal thickening, and those forming “normal” continental crust with average thickness, which require crustal flow and soft crustal rheology.

The origin of Proterozoic massif-type anorthosites: evidence from interactions between crustal xenoliths and basaltic magma

Plagioclase-rich reaction zones occur around numerous aluminous crustal xenoliths within a suite of Palaeogene sub-volcanic basic sheets on the Isle of Mull, NW Scotland. The xenoliths consist of a glassy core, containing mullite needles, generated from the melting of pelitic source rocks. Thick plagioclase mantles grew at the interface between the aluminous liquid and the enclosing basaltic magma and provide a high-level analogue for the petrogenesis of Proterozoic massif-type anorthosites. Similar interactions between mantle-derived basic magmas ponded at the base of the crust and relatively Al-rich lower crustal lithologies would result in the precipitation of large volumes of plagioclase. Anorthosite massifs were then emplaced at higher crustal levels as crystal-rich mushes within relatively juvenile Proterozoic crust. The model negates the need to crystallize large volumes of mafic minerals prior to the production of plagioclase-saturated liquids, and also accounts for the significant influence of crustal sources on the isotopic signatures of all members of the anorthosite suite.

New reflections on the structure and evolution of the Makkovikian ‐ Ketilidian Orogen in Labrador and southern Greenland

Marine seismic reflection profiles across the Early Proterozoic Makkovikian ‐ Ketilidian Orogen in the Labrador Sea region suggest that it is a doubly vergent, asymmetric orogenic belt, comparable in width to younger collisional orogens. A southeast dipping reflector package is correlated with on‐land shear zones that mark the southeastern limit of exposed reworked Archean crust and is also associated with a cryptic isotopic boundary in granites, which documents a transition from “ancient” to “juvenile” basement. This boundary is interpreted as a suture, along which juvenile Proterozoic crust has been juxtaposed over (thrust over ?) the Archean craton. A major synorogenic to postorogenic plutonic terrane to the southeast has a poorly reflective upper crust but shows strong subhorizontal reflectivity in the lower crust and Moho regions. The southeastern part of the profile correlates with metasedimentary terranes of the Ketilidian Belt in Greenland and contains low‐angle, northwest dipping, reflector packages suggestive of large‐scale crustal imbrication by thrusting. At least two broad zones of reflectivity at mantle depths (up to 16 s) are also recognized. One dips northwest below the Archean craton, but the most widespread mantle reflectivity is southeast dipping in opposition to the dominant fabric in the overlying crust. These contrasting crustal and subcrustal reflectivity patterns define a geometric “focus” beneath the orogenic belt and may provide information about subduction polarity during its development. The doubly vergent reflectivity pattern resembles images from possibly correlative Precambrian orogenic belts such as the Penokean and Svecofennian and also younger “small collisional orogens” such as the Appalachian and Pyrenean belts. It also broadly resembles some results of geodynamic crustal deformation models based on detachment and underthrusting of mantle lithosphere following collision and the squeezing of “weak” zones between rigid bounding blocks. A speculative multistage model for the belt incorporates initial northward subduction beneath the craton, shifting to later southward subduction, followed by oblique accretion of a composite arc terrane and juvenile (?) continental block. The accreted, hotter, juvenile, Proterozoic crust behaved differently than the stable, cool, Archean crust and experienced subhorizontal shearing in lower crustal and Moho regions, associated with southeast directed imbrication of the middle and upper crust by thrusting. However, the present reflectivity pattern of the orogen may also include elements related to post collisional extensional collapse.

Definition of an Archean – Proterozoic crustal suture by isotopic studies of basement intersections from offshore wells in the southern Labrador Sea

Eight hydrocarbon exploration wells in the southern Labrador Sea penetrated Precambrian basement rocks, adjacent to the course of a Lithoprobe marine seismic reflection survey. The rock types are mostly Early Proterozoic (1870–1800 Ma) calc-alkaline plutonic rocks or their metamorphic derivatives, but one example is a Middle Proterozoic (1270 Ma) anorogenic intrusion. These samples of buried basement document a transition from juvenile Proterozoic crust in the southeast (shown by εNd from +0.8 to +2.2) to ancient Archean crust in the northwest (shown by εNd from −2.7 to −7.0). However, initial Sr isotope ratios do not show such a clear pattern. The transition in εNd is similar to results of onland studies in Labrador and formerly adjacent south Greenland, and delimits the hidden edge of the North Atlantic Archean craton within the Early Proterozoic Makkovikian–Ketilidian Orogen. These results from close to the seismic profile confirm that regional reflectivity contrasts on either side of the isotopic boundary record fundamental differences between local Archean and Proterozoic crust. Southeast-dipping reflectors that broadly correspond with the boundary zone probably represent a collisional suture zone, along which juvenile terranes were probably placed structurally above the Archean craton.

Proterozoic orogens of the northeastern Canadian Shield: new information from the Lithoprobe ECSOOT crustal reflection seismic survey

As part of the Eastern Canadian Shield Onshore–Offshore Transect (ECSOOT), Lithoprobe acquired 1250 km of deep seismic reflection data along the coast of Labrador and across Ungava Bay, to image evidence of Proterozoic crustal accretion to the Archean nuclei of the Nain and Superior provinces of the Canadian Shield. The relatively pristine Archean crust of the Nain Province has low reflectivity and generally lacks systematic reflector orientations. Reworking of Archean crust on the margins of the Makkovik Province has little effect on this weak signature. In contrast, the Archean crust in the Eastern Churchill (Rae) Province appears to have been overprinted by a strongly developed, whole-crustal, easterly dipping reflection fabric, interpreted to result from Proterozoic collision of the Nain and Superior provinces in the paired New Quebec and Torngat orogens. Juvenile Proterozoic crust in the Makkovik and Grenville provinces also shows strong whole-crustal dipping reflection fabrics, interpretable as outwardly verging structures associated with collisional mobile belts. Crustal thickness varies from 35 to 45 km in Proterozoic provinces, except where thinner in areas probably affected by Mesozoic extension associated with rifting of the Labrador Sea. Dipping reflectors in the mantle are commonly associated with strong lower-crustal dipping reflections in a manner similar to that observed in some modern orogens. The ECSOOT data show that Proterozoic crust in this area has structural forms comparable with those of modern orogens and, inferentially, its tectonic development was controlled by very similar collisional processes.

Petrochemical and petrological characteristics of 1.9 Ga old volcanics in northern Sweden

Abstract Palaeoproterozoic, ∼ 1.9 Ga old, subaerial felsic to mafic volcanics of the Kiruna-Arvidsjaur Porphyry Group (KAPG) are abundant in northernmost Sweden. The felsic-dominated KAPG is situated north of a proposed north-dipping subduction zone along the boundary between the Archaean craton in the northeastern part of the Baltic Shield and juvenile Proterozoic crust in the south. The geochemistry of the volcanics indicates that plate-tectonic and volcanic-arc processes similar to recent ones occurred in northern Sweden ∼ 1.9 Ga ago. The volcanics were formed in extensional as well compressional environments. Calc-alkaline volcanics mark compression in the south and in the east. Mildly alkaline volcanics occurring in the west were formed in an extensional setting. The same rift zone gave rise to a basin filled with a thick sequence of younger sediments. On the basis of regional differences in geochemistry and petrology, the KAPG has been divided into the Kiruna, Arjeplog. Arvidsjaur and Lulea subprovinces. The volcanics in the Skellefte ore district south of the KAPG occurrence area have a volcanic-arc character and can be related to the volcanics of the Arvidsjaur subprovince immediately to the north; however, the latter are subaerial whereas the Skellefte volcanics represent a marine facies. The northerly Kiruna subprovince represents a different tectonic environment with a mafic to intermediate, trachytic volcanism that indicates extensional environments. The pattern of the different geotectonic settings indicates that the development of the ∼ 1.9 Ga old volcanics cannot be assigned solely to the action of a subduction zone dipping northwards beneath the Skellefte district.

Geological constraints on the origin of the mantle root beneath the Canadian shield

Cratonic North America is composed of a cluster of Archaean microcontinents centred on the Canadian shield, and juvenile Proterozoic crust that lies mainly buried beneath the sedimentary cover of the western and southern interior platforms. The shield is underlain by an anomalous low-temperature mantle root that is absent beneath the platform. As there appears to be no systematic difference in crustal thickness or density between the shield and the platform, the long-lived arching of the shield implies an intrinsic buoyancy imparted by the mantle root that more than offsets its colder temperature. Isotopic and seismic anisotropy data indicate an Archaean age for the mantle root, close to the time of formation of the overlying crust. The preferential development of the mantle root beneath Archaean crust is consistent with an origin by imbrication of partly subducted slabs of highly depleted oceanic lithosphere, assuming that buoyant subduction was more common in the Archaean. Formation of the mantle root was not dependent on collisional orogenesis, as has been suggested, but the Archaean cratonic mantle was sufficiently buoyant and refractory to survive later tectonic thickening. The mantle root persists beneath Archaean crust that was transected by mafic dyke swarms and subjected to short-lived episodes of post-orogenic crustal melting, but the root is reduced at mantle plume initiation sites. The partitioning of Archaean and Proterozoic crust between the shield and the platform, respectively, causes the shield to misrepresent Precambrian crust as a whole. Studies of the shield falsely conclude that a high percentage of Precambrian crust formed in the Archaean, and that the Proterozoic was characterized by epicontinental volcanism and sedimentation, and crustal ‘reworking’. Furthermore, the isotopic ratios of detritus eroded from the craton may tend to overestimate the mean age of continental crust.