The Coldwell alkaline Complex is located on the north shore of Lake Superior, and consists of three Centers representing three magmatic episodes. Center I consists of gabbro, and layered and unlayered ferroaugite syenite. Center II consists of alkalic biotite gabbro, miaskitic nepheline syenite, amphibole nepheline syenite, perthitic nepheline syenites and recrystallized nepheline syenites. Center III consists of magnesio-hornblende syenite, ferro-edenite syenite, contaminated ferroedenite syenite, and quartz syenite.
The textures and compositions of the feldspars and apatite from these three Centers were examined using cathodoluminescence (CL) and the scanning electron microscope (SEM). All of the colours described in the text refer to cathodoluminescence colours. The feldspars in Center I consist of light blue to light violet blue, optically homogeneous alkali feldspar; braid microperthite and incipient perthite; and irregular vein or patch perthite. The irregular vein patch perthite contain light violet blue cryptoperthitic patches, light blue exsolved albite and dull blue exsolved K-feldspar. During late-stage fluid-induced alteration, these feldspars were replaced by violet secondary albite and purple secondary K-feldspar; and were coarsened by a deuteric antiperthitic rim. The Fe-rich antiperthitic rim consists of deep red secondary albite and brown secondary K-feldspar. The later dominates in the upper series of the layered and unlayered syenites. The homogeneous alkali feldspar crystallized at high temperatures ≤700°C, exsolved into microperthite at 650-550°C and formed irregular vein and patch perthite at 520-420°C. Late-stage fluid-induced replacement and deuteric coarsening occurs at relatively low temperatures 500-300°C.
The feldspars in Center II consist of light blue or light bluish grey oligoclase; light violet blue homogeneous alkali feldspar; and irregular vein or patch perthite. A few oligoclase crystals are mantled by a light blue alkali feldspar rim. Most of the irregular vein and patch perthite contain light blue exsolved albite and dull blue K-feldspar host; and some of them contain light violet blue to light violet unexsolved alkali feldspar patches in the core, and violet blue to dull blue cross hatched microcline along the margin. During late-stage fluid-induced deuteric alteration, the alkali feldspar was replaced by light violet blue to light violet secondary albite or dull blue to dark brown secondary K-feldspar along the margins; and the alkali feldspar was coarsened by a red to deep red secondary albite rim. The oligoclase crystallized at high temperatures 800-750°C, and the homogeneous alkali feldspar crystallized at ³710-620°C. The perthite texture and the unexsolved alkali feldspar patches may have formed at 600-450°C. The K-feldspar host may have transformed to the cross-hatched microcline at ≈300°C. The deuteric coarsened and replaced secondary feldspar formed at low temperatures <350°C.
In the recrystallized nepheline syenites of Center II, the feldspar crystals are usually mantled with a relict core and a light violet blue recrystallized alkali feldspar rim. The relict cores usually are a light blue albite or light violet homogeneous alkali feldspar.
The feldspars in Center III consist of zoned plagioclase crystals with light greenish blue andesine-oligclase cores and light violet blue to light blue oligoclase-albite rims; light blue to blue homogeneous alkali feldspar, K-rich feldspar, braid microperthite and oscillatory zoned alkali feldspar; irregular vein perthite consisting of light blue to light violet exsolved albite and dull blue K-feldspar host; and regular to irregular vein antipperthite consisting of dull blue to dark blue exsolved K-feldspar and dull red to deep red albite host. Most irregular vein antiperthite grains reveal alternating dull red, violet and deep red oscillatory zones. During late-stage fluid-induced alteration, most light blue to blue original alkali feldspar were coarsened by a deuteric antiperthite rim which consists of red secondary albite and dark brown secondary K-feldspar. The plagioclase crystallized at high temperatures about 900-750°C, the homogeneous alkali feldspar crystallized at ³725-520, and the K-rich feldspar crystallized at 580-450°C. The microperthitic texture may have formed at 650-420°C, and the perthite and antiperthite textures may have formed at non-equilibrium temperatures about 520-350°C. The deuteric coarsened antiperthitic rim formed at temperatures <350°C.
The apatite crystals in the Coldwell complex exhibit a variety of CL textures which include: (1) uniform light pink to pink grains; (2) growth zones with a small brownish pink core and light pink to pink rims; (3) light pink to pink alternating oscillatory zones; (4) mantled grains with light pink to pink cores and brownish pink reaction rims. Some of the reaction rims are overgrown by light pink and yellow secondary apatite.
The light pink and pink CL colours in the apatite crystals are dominantly caused by Eu2+ activation, the brownish pink CL colour is dominantly caused by Dy3+, Sm3+ and Pr3+, and the yellow luminescent secondary apatite is characteristic of Dy3+ and Tb3+ activation. The brownish pink apatites have higher contents of total REE and Si than the light pink apatites. The yellow or light pink secondary apatites have relatively low contents of total REE and Si. The apatite crystals in the Coldwell complex are characterized by LREE-enrichment which is caused by the coupled substitution of 2Ca2++P5+X(REE3++REE2+)+Si4+. The Si content in the apatite crystals increase from Center I through Center II to Center III.
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