Jacupiranga Complex
Geological Setting
Mainly driven by the Atlantic Ocean opening, the last activation stage of the South American Platform comprises episodes of both tholeiitic and alkaline magmatism. In the southern part of the platform, the tholeiitic basaltic activity gave rise to the Paraną Magmatic Province (137 Ma), one of the largest continental food basalts in the world, accompanied by minor intrusive activity. Its alkaline counterpart occurs as intrusive bodies with a wide range of composition (SiO2-satured and undersatured, basaltic rocks, carbonatites and kimberlites), yielding early to late cretaceous ages. In the central-southeastern part of the platform (Fig.1A), the alkaline occurrences are distributed along the margins of the intracratonic Paraną basin. The detailed tectonic framework controlling the distribution of the alkaline activity in the eastern margin of the basin is still controversial, despite being clearly associated with the basin boundaries.It is noteworthy that these igneous bodies are aligned along the Lancinha-Cubatćo shera zone (Fig.1A formed during the Gondwana amalgamation. There is no general agreement on neither the mechanical nor the petrogenetic relation between these tectonic features and the genesis of the alkaline complexes. Nonetheless, the southern and central intrusions are usually related to the Ponta Grossa Arch (Riccomi et al., 2005), a NW-SE trending structure enhanced by swarm of basaltic dikes (Fig.1B). Among the four lineaments controlling the Punta Grossa Arch structural framework (Gupiara, São Jeronimo-Curiuva, Rio Alonso and Rio Piquiri), the southern sector of the Gupiara and São Jeronimo-Curiuva lineaments present some alkaline and alkaline carbonatite intrusions aligned parallel to their axis.
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Fig. 1 A Alkaline occurrences along the Paraną Basin margins at the central-southeastern part of the South America platform: 1) Late Cretaceous Baru Basin; 2) Early Cretaceous Paraną Magmatic Province; 3) Late Ordovician to Early Cretacrous Paraną Basin; 4) alkaline units (diamonds = Permina-Triassic, squares = Early Cretaceous, triangles = Late Cretaceous, circle = Paleogene); 5) Axes of main arch; 6) Major fracture zones (GP = Guapiara; JC = Sćo Jeronimo-Curiuva; RA = Rio Alonso; RP = Rio Piquiri); 7) Lancinha-Cubatão shera zone. B Detailed tectonic setting of Cretaceous alkaline magmatism in the Punta Grossa Arch. (a) and (b) dikes; (c) alkaline complexes (BIT = Barra do Itapirãua, BN = Banhandão; BT = Barra do Teixeira, CN = Cananeia, IP = Ipanema, IT = Itapirapuã, JAC = Jacupiranga, JQ = Juquirą, MP = Mato Preto, PAR = Pariquera-Arcu and TU = Tunas). Modified from Chmyz, Luanna, et al (2017).
The Jacupiranga Complex
The Jacupiranga Igneous Complex (JIC) was discovered in 1877 by H. E. Bauer. In 1891, Derby described the area of carbonatite outcrop as an iron ore occurrence, due to its high magnetite content. In 1954, Melcher provided the first detailed mapping and petrographic descriptions of the rocks in the complex, recognized the carbonatites, and proposed they had been intruded in two distinct events. The weathered carbonatites, on the top of a smoothed hill, were already being mined at the time.The Jacupiranga Igneous Complex is located in the south-eastern region of Brazil, 230 km from São Paulo. The Complex is an intrusive body with a described outcropping area of approximately 65 km2 (Ruberti et al., 2000). It has an elongated ellipsoidal shape in plain view, with its longest dimension oriented in an NNW direction (0.5 x 6.7 km). The Complex is largely formed by two ultramafic intrusions: dunites in the north, and clinopyroxenites (jacupirangite) in the south. Subordinate alkaline rocks and other lithologies are present in the complex in smaller volumes.
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Fig. 2: Geological sketch map and cross section of the Jacupiranga complex. Modified from Salvioli-Mariani, E., et al (2014).
The carbonatites exclusively intrude the clinopyroxenite and outcrop in an area of approximately 0.7 km2. They largely correspond to a sequence of pipe-like intrusions with sub-circular cross sections, and related dikes. Numerous rounded, large xenoliths of the clinopyroxenite (from few centimeters up to about four or five meters) occur in the carbonatite. Among carbonatitic phases, there are different compositions (calcio-carbonatites to magnesium-carbonatites) forming independent plugs, dykes and dyke swarms (Fig.2).
Gaspar (1989, 1991) grouped the Jacupiranga Igneous Complex rocks into three categories:
1) Ultramafic rocks: dunites, clinopyroxenites, carbonatites, lamprophyres.
2) Nepheline-bearing rocks: ijolites, malteigites.
3) Feldspar-bearing rocks: gabbros, diorites, monzonites, syenites.
Dunites
Dunites are covered by a thick lateritic cover, exceeding 40 m, due to local prevailing climatic condition. Olivine partially altered by serpentine dominate the dunites. Al- and Ti-bearing spinels, iron hydroxide and carbonate may be present.
Clinopyroxenite (jacupirangite)
Jacupiranga clinopyroxenite are typically, medium to coarse-grained, layered cumulates. Magnetite clinopyroxenites are by far the dominant lithotype and form the largest part of the unit. They mainly consist of cumulus diopside (> 80%) and interstitial magnetite. Scarce perovskite, phlogopite, apatite, kaersutite and interstitial carbonate are also present.
Carbonatite
carbonatites (sövites), intruding the clinopyroxenite body, show variable textural and mineralogical features. They are coarse- to medium-grained (grain size between 0.3 and 7 mm) and sometime banded. The main mineral assemblage of Jacupiranga carbonatites include calcite, dolomite, apatite (5 to 35%) and magnetite; olivine, phlogopite, ilmenite, pyrochlore are accessory phases. Apatite is the most important accessory phase and make Jacupiranga one of the most significant Brazilian carbonatite-hosted phosphate ores.
Ijolite rocks
Ijolite rocks forms the highest hill of the Jacupiranga complex (Morro Grande hill TILDE 290 m). That series of rocks consists mainly of ijolites and minor malteigites and some rare urtites. Ijolites are composed of diopside (20-40%), nepheline (20-40%), phlogopite, magnetite (5-10%), kalsilite, apatite and perovskite.
Feldspar-bearing rocks
All the Feldspar-bearing rocks from Jacupiranga occur as veins, dike swarm, or minor intrusions. They can be classified into three major group: diorites, monzonites and syenite.
Formation of the Jacupiranga Igneous Complex
Reconstruction of magmatic intrusions at depth is not straightforward, since in is well know their highly variable mode of emplacement an geometry that depend on a number of factors such as composition, size and depth of plutonic bodies, nature of the host rocks, as well as tectonic regime. In particular, for many ring alkaline complexes, the intrusion mechanism cannot be explained in terms of classic magma chamber where a homogeneous magma body undergoes fractional crystallization and crystal settling leading to mafico-ultramafic cumulates at the bottom and felsic differentiates at the top. An increasing number of papers have shown that many shallow (< 5 km depth) small-sized mafic intrusions, differently from major stratiform complexes, were emplaced in from of soucer- or cup-shaped plutonic bodies (Hansen and Cartwright, 2006; ODriscoll at al., (2008).Experiments by Mathieu et al., (2008) and Galland et al., (2009, 2004) demonstrated that cup-shaped and cone-sheet intrusions could result by shear faulting at the tip of nearly vertical feeding conduit under suitable stress regime and viscosity (Fig.3). In these complexes, the initiation of magma emplacement is plausibly characterized by multiple injections of dyke and sills, which progressively amalgamate at shallow level in the host rock. During such processes the combined effect of progressive crystallization and continuous magma inflation induce formation of mafic crystal mushes (olivine and(or pyroxene) at the center of the intrusion, whereas the remaining differentiates melts are forced to escape laterally through cone-sheet.
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Fig. 3: Model of magma intrusion (T1 =time 1), propagation and cup-shaped cumulate growth (T2 = time 2). Modified from Beccaluva, Luigi, et al. (2017).
The Jacupiranga complex consist of two diachronous silicate intrusions: an older dunite-gabbro-syenite body and a younger clinopyroxenite-ijolite body, injected by carbonatitic core (< 1% volume). The two silicate intrusions, probably generated from different parental magma which evolved, under nearly closed system conditions, in two distinct zonally arranged plutonic bodies that grew incrementally from independent feeding systems (Fig.4). The first intrusion was generated by alkaline to mildly alkaline parental basalts that initially led to the formation of a dunitic adcumulate core, surrounded by gabbroic cumulates, in turn injected by sub-annular syenites intrusives and phonolite dykes. The second intrusion, formed from nephelinitic parental melts that gave rise to clinopyroxenitic cumulates in turn surrounded, and partially cut by malteigite-ijolite-urtite cumulates.
Carbonatites, representing the last magmatic event of the complex, outcrop at the center of the second intrusion, and possibly intruded through the same feeding conduit. A possible origin for Jacupiranga carbonatites at mantle depth is suggested by the absence of carbonatite ocelli in the silicate counterparts of the intrusion, and by melts and fluids inclusions in apatites, indicating trapping depth in the range 30-60 Km.
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Fig.4: Cartoon illustrating the inferred mode of the Jacupiranga complex. A: first dunite-gabbro-syenite intrusion resulting from continuous infiltration of alkali-basaltic parental magma and formation of cumulus dunites at the center of the intrusion and, subsequently, to gabbroic-syenitic differentiates. (AG = Alkali gabbro; SD = Syenodiorite; Du = Dunite) B: second clinopyroxenite-ijolite intrusion partially cutting the southern part of the first intrusion. The nephelinitic parental magma give rise to clinopyroxenitic cumulates constituting the majority of the intrusion and cup-shaped layered body of ijolite-melteigite-urtite intrusives. The carbonatite intrudes the clinopyroxenite cumulates possibly through the same feeding conduit (Cp = Clinopyroxenite; Ij = Ijolite; Carb = Carbonatite). Modified from Beccaluva, Luigi, et al. (2017).
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Cajati mine in the carbonatite intrusion. From Registro diario Cajati
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Magnetite (dark) in a carbonatite from Jacupiranga. from E-rocks
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Magnetite (dark) in a carbonatite from Jacupiranga. from E-rocks
Bibliography
• Alva-Valdivia, Luis M., et al. "Rock magnetism and microscopy of the Jacupiranga alkaline-carbonatitic complex, southern Brazil." Earth, planets and space 61.1 (2009): 161-171.
• Beccaluva, Luigi, et al. "The alkaline-carbonatite complex of Jacupiranga (Brazil): magma genesis and mode of emplacement." Gondwana Research 44 (2017): 157-177.
• Chmyz, Luanna, et al. "Ar-Ar ages, Sr-Nd isotope geochemistry, and implications for the origin of the silicate rocks of the Jacupiranga ultramafic-alkaline complex Brazil)." Journal of South American Earth Sciences 77 (2017): 286-309.
• Salvioli-Mariani, E., et al. "Late veins of C3 carbonatite intrusion from Jacupiranga complex (Southern Brazil): fluid and melt inclusions and mineralogy." Mineralogy and Petrology 104.1-2 (2012): 95-114.