Quartz monzonite
Quartz monzonite is a plutonic igneous rock characterized by roughly equal proportions of alkali feldspar (such as orthoclase or microcline) and plagioclase feldspar, along with 5–20% quartz in the QAPF modal classification, distinguishing it from granite (which has more quartz) and monzonite (which has less or none).[1] This felsic to intermediate rock typically includes mafic minerals like biotite, hornblende, or augite, as well as accessory phases such as zircon, magnetite, ilmenite, and apatite, resulting in a phaneritic texture with medium- to coarse-grained crystals.[1][2] Formed through the slow crystallization of silica-rich magma in the Earth's crust, often in subduction zones or continental magmatic arcs, quartz monzonite intrudes into surrounding rocks as batholiths or stocks, cooling over millions of years to produce durable plutons.[2] Its composition reflects a balance of silica (around 60–70%), alumina, and alkalies, with more calcium than typical granites but less than diorites.[3] Physically, it exhibits high hardness (6–7 on the Mohs scale), low porosity, and resistance to weathering, appearing in shades of gray, pink, or reddish-brown depending on mineral content and oxidation.[2] Quartz monzonite occurs widely in orogenic belts and ancient cratons, with notable examples in the Sierra Nevada Batholith (California, USA), the Coast Mountains (British Columbia, Canada), Stone Mountain (Georgia, USA), and Precambrian intrusions like those in Black Canyon of the Gunnison National Park (Colorado, USA), where it dates to 1.2–1.4 billion years ago.[2][3] Due to its strength and aesthetic appeal, it is quarried as dimension stone for construction, countertops, flooring, and monuments, serving as a key material in both historical and modern architecture.[2]Definition and Classification
Petrographic Definition
Quartz monzonite is an intrusive igneous rock classified modally within the QAPF system of the International Union of Geological Sciences (IUGS), where it occupies field 8* on the ternary diagram for plutonic rocks lacking feldspathoids. This field is defined by quartz comprising 5-20% of the total Q + A + P modal fraction, with plagioclase feldspar (typically oligoclase to andesine, An10-50) and alkali feldspar (primarily orthoclase or microcline) each ranging from 35-65% of that fraction.[4] The equal or near-equal proportions of the two feldspars distinguish it as a monzonitic rock, while the presence of quartz sets it apart from more mafic or syenitic compositions. The rock exhibits a phaneritic texture, characterized by coarse-grained, interlocking crystals generally larger than 1 mm that are visible to the naked eye, resulting from the slow cooling of magma in shallow to deep crustal plutonic environments.[5] This texture often appears equigranular, though porphyritic variants with larger feldspar phenocrysts in a finer matrix can occur. Minor mafic minerals, such as biotite or amphibole, may impart a subtle color variation but do not alter the primary felsic character.[4] In contrast to monzonite, which falls into the same QAPF field but with less than 5% quartz, quartz monzonite requires at least 5% quartz for its designation, reflecting a modestly higher silica content that influences its plotting position on the IUGS diagram.[4] This threshold ensures precise differentiation in petrographic analysis, emphasizing modal mineralogy over chemical norms.Comparison to Related Rocks
Quartz monzonite is differentiated from monzonite primarily by its quartz content, which ranges from 5% to 20% in modal composition, compared to less than 5% quartz in monzonite; this addition of quartz shifts quartz monzonite toward a more felsic character while maintaining roughly equal proportions of plagioclase and alkali feldspar (both 35-65%).[6] In contrast to granodiorite, quartz monzonite features a higher proportion of alkali feldspar (35-65%) relative to plagioclase (35-65%), whereas granodiorite is dominated by plagioclase (65-90%) with only 10-35% alkali feldspar, often including more calcic plagioclase varieties.[6] Relative to granite, quartz monzonite contains significantly more plagioclase feldspar (up to 65%) and less alkali feldspar, as granite is characterized by over 65% alkali feldspar and typically under 35% plagioclase, alongside 20-60% quartz in both but with distinct feldspar ratios.[6] The following table summarizes the key modal percentage ranges for quartz, alkali feldspar (A), and plagioclase (P) in these rocks according to the International Union of Geological Sciences (IUGS) QAP classification for plutonic igneous rocks:| Rock Type | Quartz (Q) | Alkali Feldspar (A) | Plagioclase (P) |
|---|---|---|---|
| Quartz Monzonite | 5-20% | 35-65% | 35-65% |
| Monzonite | 0-5% | 35-65% | 35-65% |
| Granodiorite | 20-60% | 10-35% | 65-90% |
| Granite | 20-60% | 65-90% | 10-35% |
Composition
Mineral Constituents
Quartz monzonite is characterized by a mineral assemblage dominated by felsic components, with plagioclase feldspar typically comprising 35-65% of the rock and consisting of sodic varieties such as oligoclase to andesine (An10-An30).[8][4] Alkali feldspar, often in the form of orthoclase or microcline, also constitutes 35-65% and occurs in roughly equal proportions to plagioclase, forming the primary framework of the rock.[8][4] Quartz is an essential mineral, ranging from 5-20% and typically appearing as anhedral, interstitial grains that fill spaces between the larger feldspar crystals.[1][4] Mafic minerals are subordinate but contribute to the rock's color and structure, with biotite being the most common at 5-15% and occurring as dark, pleochroic flakes that impart a gray to brownish hue.[9][1] Hornblende, a calcic amphibole, is present in amounts up to 10% in some variants, forming euhedral to subhedral prisms, while pyroxene such as augite appears occasionally in trace quantities.[9][1] Accessory minerals include magnetite (opaque oxides), apatite, zircon, and titanite, each typically less than 2-5% and occurring as small, euhedral crystals disseminated throughout the matrix; minor amounts of sphene (another name for titanite) or allanite may also be present.[9][1] Common mineral intergrowths feature perthitic textures in alkali feldspar, where exsolved albite lamellae create a striped appearance, and myrmekitic intergrowths at the boundaries between plagioclase and quartz, consisting of vermicular quartz in a plagioclase matrix.[10] Textural variations in quartz monzonite are generally equigranular and medium- to coarse-grained, but porphyritic textures occur rarely, with phenocrysts of feldspar up to several centimeters embedded in a finer groundmass.[8][10]Chemical Characteristics
Quartz monzonite exhibits a felsic to intermediate bulk chemical composition dominated by silica, with typical SiO₂ contents ranging from 65 to 75 wt%, reflecting its quartz-bearing nature.[11] Al₂O₃ concentrations are generally 14-18 wt%, primarily derived from feldspar components, while alkali oxides total 7-9 wt% (Na₂O + K₂O), and CaO falls between 2-5 wt%. Compared to more mafic rocks, quartz monzonite shows notably lower MgO (typically <2 wt%) and total iron oxides (FeO + Fe₂O₃ <5 wt%), underscoring its evolved, silica-rich character.[11][12] The following table presents an average major oxide composition for quartz monzonite, based on compiled analyses (values in wt%):| Oxide | Average (wt%) |
|---|---|
| SiO₂ | 68.65 |
| TiO₂ | 0.54 |
| Al₂O₃ | 14.55 |
| Fe₂O₃ | 1.23 |
| FeO | 2.70 |
| MnO | 0.08 |
| MgO | 1.14 |
| CaO | 2.68 |
| Na₂O | 3.47 |
| K₂O | 4.00 |
| P₂O₅ | 0.19 |