|Author:||Charette and Beaudette, 2018|
|Geological province:||Churchill Province|
|Geological subdivision:||Ungava Orogen / Narsajuaq Lithotectonic Domain|
|Lithology:||Intermediate to mafic granulitic rocks|
Table des matières
The Pingasualuit Complex was introduced by Charette and Beaudette (2018) to describe intermediate to mafic foliated granulitic rocks forming a large part of the Narsajuaq Domain. These were previously included in the older plutonic suite (pPNAR1) and unit pPNAR2 of the younger plutonic suite, described by St-Onge and Lucas (1992) and St-Onge et al. (1992).
The Pingasualuit Complex consists primarily of banded or locally gneissic hypersthene quartz diorite and hypersthene diorite (nApgs2), as well as metric to kilometric horizons of mesocratic gabbronorite and hypersthene diorite (nApgs1). The majority of this complex is characterized by a paragenesis characteristic of the granulite facies. In places, mafic to ultramafic klippes at the amphibolite facies, possibly retromorphosed, are included in subunit nApgs1a.
Unit nApgs1 consists of klippes and hectometric to kilometric strips of mesocratic and granoblastic gabbronorite and hyperstene diorite usually present in the main unit of the Pingasualuit Complex (nApgs2). These mafic rocks are dark greenish grey to brownish black in altered surface. The rock is heavily recrystallized, fine to medium grained and contains 50-65% ferromagnesian minerals. It also has a homogeneous, speckled or foliated to banded appearance. Banding is produced by a variation in the modal proportion of ferromagnesian minerals (35-75%). On the other hand, the speckled appearance is due to millimetric to centimetric clusters of ferromagnesian minerals and recrystallized plagioclase clusters <1 cm. Leucosomes occuring as 2 mm to 10 cm diffuse clusters form up to 10% of the rock. They are slightly coarser grained than the matrix. They are recrystallized, oriented to foliation and include isolated crystals or clusters of clinopyroxene, orthopyroxene and hornblende. Locally, garnet clusters are observed. In thin sections, minerals are strongly recrystallized. Hornblende and clinopyroxene are the principal ferromagnesian minerals, followed by orthopyroxene and biotite. Clinopyroxene is commonly amphibolitized and orthopyroxene is partially replaced by serpentine and oxides. Hornblende marks the foliation and occurs as poikiloblastic crystals in some samples. Very few accessory minerals are observed (opaque minerals, apatite and carbonates).
Pingasualuit Complex 1a (nApgs1a): Gabbro, Diorite and Associated Ultramafic Rocks
Subunit nApgs1a consists of klippes of intermediate to mafic rocks without orthopyroxene in the vicinity or within units of the Pingasualuit Complex. Unlike rocks of unit nApgs1, rocks of subunit nApgs1a do not have the brownish colour typical of altered orthopyroxene rocks. Their alteration patina ranges from greenish black to dark green. In thin section, the presence of serpentine clusters and orthopyroxene cores locally preserved in hornblende crystals suggests that they are granulitic rocks retromorphosed to the amphibolite facies. With the exception of their colour, the appearance of these rocks in outcrop differs little from the rest of unit nApgs1. They are banded or homogeneous and foliated. A speckled facies with clusters of ferromagnesian minerals and small centimetric plagioclase crystals is also observed. In some places, in this facies, ferromagnesian mineral clusters are centimetric and joint, giving a recrystallized cumulate appearance. Ultramafic horizons and boudins are locally present in gabbro and diorite of subunit nApgs1a. A few fractured olivine crystals (<10%) partially replaced by iddingsite and magnetite are observed in thin sections of a websterite.
No orthopyroxene was observed in the area north of the Naujaat Shear Zone. Lithologies in this area are foliated, homogeneous and have, in places, discontinuous leucosomes indicating partial melting. Microscopic observations identify hornblende, actinolite and clinopyroxene as the main mafic minerals. Clinopyroxene is variably amphibolitized and biotite replaces hornblende in fracture and cleavage planes.
Unit nApgs2 includes hypersthene quartz diorite and hypersthene diorite, fine to medium grained, usually banded and granoblastic. They form the majority of the Pingasualuit Complex. These rocks are greenish to brownish grey in altered surface and have a foliated to gneissic fabric, as well as banding marked by the presence of mafic mineral laminae and millimetric to centimetric leucocratic bands. These are brownish to whitish beige in altered surface and generally vary from a quartz dioritic to tonalitic composition, even granitic locally. Some of these bands are fine grained, conformable to foliation and in diffuse contact in the matrix. They occur as lenticular clusters in places and commonly include orthopyroxene, clinopyroxene and magnetite crystals. These thin bands, interpreted as leucosomes derived from melting of intermediate granulitic rocks of the Pingasualuit Complex, account for 5-20% of the rock. Other bands are slightly coarser grained than the fine-grained matrix, and they are subconformable to or cutting foliation and in sharp contact. In places, these coarser-grained bands include quartz bands. At the outcrop scale, banding is also marked by compositional variations produced by variation of ferromagnesian mineral contents (10-40%) within quartz diorite or hypersthene diorite. Centimetric to metric horizons of nApgs1 gabbronorite are present in sharp or diffuse contact with granulitic intermediate rocks. These horizons are usually more competent and less banded than host rocks.
In diffuse contact with the banded main facies, a second facies of hypersthene quartz diorite is medium grained, homogeneous and speckled. This facies is characterized by millimetric clusters of mafic minerals and slightly coarser plagioclase crystals producing the speckled appearance. The modal proportion of ferromagnesian minerals (10-20%) is generally lower in this facies than in the main facies and the granoblastic texture is less developed.
Petrographic studies and geochemical analyses indicate that the composition of unit nApgs2 varies from quartz dioritic, dioritic and quartz monzodioritic to tonalitic or monzodioritic locally. In places, these variations are observed in diffuse centimetric bands within the same sample. When present, K-feldspar is distributed heterogeneously as fine crystals disseminated in the matrix, as inclusions in plagioclase or as fine laminae. Plagioclase forms, in places, millimetric (3- 7 mm) rounded crystals that are coarser than the fine-grained matrix, while mafic minerals occur as discontinuous millimetric laminae and are disseminated in the matrix. In thin sections, minerals are moderately to well recrystallized in general. Hornblende and pyroxene are the main ferromagnesian minerals followed by biotite. The modal proportion of these minerals varies by band. Hornblende either occurs as recrystallized even-grained crystals or as crystals with a poikilitic core and a rim without inclusions. Pyroxene crystals are commonly elongated along primary foliation. Specifically, orthopyroxene is fractured and partially replaced by iddingsite and oxides. Biotite forms rosettes with quartz intergrowth and is observed as a partial replacement for orthopyroxene or hornblende. Locally, garnet is present in small stocky crystals in equilibrium contact with hornblende. Epidote locally forms small stocky crystals associated with mafic laminae or clusters. In places, chlorite is replacing hornblende, biotite and garnet in fractures.
Pingasualuit Complex 2a (nApgs2a): Intermediate Granulitic Gneiss
Subunit nApgs2a consists of granulitic banded sequences of felsic to intermediate composition whose fine grain size and compositional variations make the origin of the protolith uncertain. These banded sequences may be paragneiss, although they may include orthogneiss. In outcrop, these rocks have a brownish to rusty alteration patina and are friable. In fresh exposure, they are medium to medium-dark greenish grey. Straight-lined banding is produced by the presence of 5-20% millimetric to centimetric leucosomes with biotite ± garnet ± orthopyroxene rims. Also, several outcrops are characterized by alternating horizons showing variations in the proportion of mafic minerals and the presence or absence of garnet. Some horizons are very rich in quartz (>70%), while others are very rich in garnet (up to 40%). Locally, horizons of diatxite with biotite-garnet schlierens are observed within granulitic gneiss.
Like unit nApgs2, rocks of subunit nApgs2a exhibit variations in the composition of diffuse centimetric bands of tonalite, granite, quartz diorite and locally quartz monzodiorite. Mafic minerals are disseminated or occur as fine laminas and account for 10-30% of the mineralogy. Biotite and orthopyroxene are the principal mafic minerals. In places, the latter is partially replaced by iddingsite and oxides or hornblende. Hornblende and garnet crystals are present locally. Garnet occurs as porphyroclasts and is locally poikilitic. Several indications of partial melting were observed in thin section such as irregular quartz films in the matrix and inclusions in garnet, the shape of which is dictated by its crystalline structure.
Hypersthene quartz diorite and hypersthene diorite of unit nApgs2 account for most of the granulitic rocks present in the Narsajuaq Domain. They form kilometre-thick strips and klippes, elongated and folded in the E-W regional fabric. They include centimetric to metric layers or enclaves of mesocratic gabbronorite and hypersthene diorite. They also form kilometric strips and klippes that have been grouped into unit nApgs1. A significant number of these intermediate to mafic rocks is present near the contact with the Kovik Lithotectonic Domain. The orthopyroxene-free facies of subunit nApgs1a is present in significant proportion north of the Naujaat Shear Zone, where it forms kilometric klippes in association with the Saint-Hélène Complex.
Two hypersthene quartz diorite samples were collected by Charette and Beaudette (2018) in the Cape Wolstenholme area. In both cases, U-Pb analyses give an age range from Neoarchean to Paleoproterozoic. Davis and Sutcliffe (2018) divided zircons into two populations on the basis of Th/U ratios. The protolith is Neoarchean, and Paleoterozoic metamorphism resulted in Pb loss via diffusion.
|Unit||Sample Number||Isotopic System||Mineral||Crystallization Age (Ma)||(+)||(-)||Metamorphic Age (Ma)||(+)||(-)||Reference(s)|
|nApgs2||2017-CT-5021A||U-Pb||Zircon||>2773||–||–||1841||24||24||Davis and Sutcliffe, 2018|
|nApgs2||2017-MP-1013A||U-Pb||Zircon||>2609||–||–||1880||9||9||Davis and Sutcliffe, 2018|
Intermediate to mafic granulitic rocks of the Pingasualuit Complex appear to be one of the oldest units in the Narsajuaq Domain. These rocks are injected bed by bed and cut by leucocratic enderbite intrusions assigned to the Navvaataaq Suite (nAnav) and by foliated to massive granitoids of the Sanningajualuk Suite (pPsnn). They are also observed as enclaves in these suites.
Does not apply.