Sean O'Brien MSc thesis abstract

Thesis Title: 
Petrology of the Crystal Lake Gabbro and the Mount Mollie Dyke, Midcontinent Rift, Northwest Ontario

The Crystal Lake Gabbro (CLG) is a Y-shaped, up to 750 m wide, layered mafic intrusion with a 5 km long northern limb and a 2.75 km long southern limb, with localized Cu-Ni and Cr mineralization. The Mount Mollie Dyke (MMD) is an arcuate, 60 to 350 m wide, macrodyke that lies on trend east of the CLG and extends for 35 km toward Lake Superior. Both intrusions are part of the 1.1 Ga Midcontinent Rift (MCR) and were emplaced into the Paleoproterozoic Rove Formation of the Logan Basin, approximately 50 km south of Thunder Bay. Current U-Pb age determination implies a ~10 m.y. age difference with CLG being formed at ~1100 Ma and the MMD being formed at ~1109 Ma. However, this age difference is at odds with both intrusions being normally polarized (an attribute of MCR rocks younger than 1102 Ma) and their being on trend with each other. This study seeks to determine whether the two intrusions may be petrogenetically linked by evaluating the petrography, geochemistry, mineral composition, and sulphur isotopes of samples collected from drill core.

The CLG profiled in a drill core from its southern limb can be broadly divided into Upper, Main, and Lower Zones with further subdivisions of the Main and Lower Zones based largely on geochemistry. The Lower Zone occurs between two xenoliths of an early MCR (~1115 Ma) plagioclase porphyritic Logan Sill diabase. The Lower Zone consists of subophitic to ophitic troctolite, augite troctolite, and olivine gabbro and can be subdivided into an upper and basal marginal subzone as well as an interior subzone. Both marginal subzones host disseminated sulphides with the basal margin also containing Cr-spinel seams. An overall bottom-up-directed fractional crystallization of the Lower Zone is suggested by the progressive decrease in Fo content of olivine, Mg# of clinopyroxene, and whole-rock MgO upsection. Above the upper Logan Sill xenolith, the Main Zone similarly consists of subophitic to ophitic troctolite, augite troctolite, olivine gabbro, and gabbro. Petrography, lithogeochemistry, and mineral composition was used to subdivide the Main Zone into five subzones: a basal marginal subzone, upper margin subzone, and three interior cycles that display cryptic variations indicative of fractional crystallization and magma recharge events.  Like the margins of the Lower Zone, the Upper Zone as well and the basal marginal subzone of the Main Zone contain disseminated sulphides and Cr-spinel, and are characterized by relatively high Fo content olivine and low incompatible trace element concentrations.  These mineralized zones are interpreted to have crystallized from the same initial pulse of magma into the CLG, which was sulfide- and Cr-spinel-saturated. Cyclical cryptic variations in the internal subzone of the Main Zone are interpreted to indicate upward directed fractional crystallization, interrupted by emplacement of additional magma pulses into the core of the intrusion. All rocks of the Main Zone are olivine and plagioclase orthocumulates indicating that fractional crystallization was not particularly efficient (i.e., did not experience a strong segregation of cumulus minerals from the parental magma).  The lack of Cr-spinel in the interior and upper marginal subzones of the Main Zone further indicates that subsequent magma pulses either were more evolved than the original  parental  magma or were volumetrically subordinate to  the evolved magmas that resided in the chamber. Throughout the evolution of the CLG, the differentiation of the magma was limited as it did not result in clinopyroxene and Fe-Ti oxide becoming cumulus phases. This was likely due to magmatic recharge and inefficient fractional crystallization.

Texturally and geochemically, the MMD can be broadly divided into an Upper and Main Zones, with a subdivision of the Main Zone into an upper and lower sequence and a pegmatitic segregation subzone. The Upper Zone consists of ferrodiorite and likely represents the end product of extensive fractionation. The Main Zone is characterized by troctolite, augite troctolite, olivine gabbro, and gabbro with MgO, CaO, Al2O3, and Ni concentrations decreasing upwards and SiO2, TiO2, K2O, Na2O, P2O5, and incompatible trace element concentrations increasing, consistent with bottom-up fractional crystallization. Strong differentiation of the MMD magma is indicated by the habit change of clinopyroxene from ophitic (intercumulus) to granular (cumulus), which is the basis for the subdivision of the lower and upper sequences. The lower sequence of the Main Zone also hosts a 24 m thick interval containing 1 to 2 m wide gabbroic pegmatite layers. These pegmatites are interpreted to be the result of localized enrichment of magmatic volatiles.

The presence of an evolved core in the MMD surface expression, coupled with the mineral composition of olivine, plagioclase, and clinopyroxene, remaining at relatively constant Fo, An, and Mg# values, respectively, below the pegmatitic layers suggests that there was some degree of lateral crystal fractionation as well as bottom up fractionation. The well-defined fractionation sequence as well as an absence of abrupt geochemical changes suggests that the MMD fractionally crystallized from a single pulse.

Liberation of external sulphur from the surrounding Rove Formation, is suggested by the greater than mantle S/Se values as well as δ34S values between +4.0 and +21.0‰ of the sulphides within the CLG.  The addition of external sulphur evidently resulted in sulphur saturation during initial emplacement of the CLG magmas.  Primitive mantle normalized multi-element diagrams and trace element ratios provide supporting evidence for a localized shallow level of crustal contamination, as well as a deeper more widespread contamination component of both the CLG and MMD magmas.

The estimated parental magma compositions and average primitive mantle normalized trace element concentrations of the CLG and MMD suggest that they shared similar, if not the same, magma source. The CLG parental magma was slightly more evolved than the MMD suggesting that the magmas were sourced from a fractionating staging chamber. The estimated parental magma compositions of the CLG and MMD closely resemble those of the Layered Series intrusions of the Duluth Complex, supporting previous speculation that the CLG may be a satellite intrusion of the Duluth Complex. Despite current geochronology data to the contrary, the results of this study strongly suggest that the CLG and the MMD are petrogenetically linked, if not parts of the same intrusive system.