Linkages between the southern Patagonia Pre-Permian basements : new insights from detrital zircons UPb SHRIMP ages from the Cerro Negro District

The Patagonian basement rocks are dominated by Precambrian to Early Paleozoic metamorphic rocks intruded by Paleozoic granitoids. Recently discovered basement rocks in the Cerro Negro District are characterized mainly by quartz-muscovite-chlorite schists; the metamorphic grade reaches greenschist facies (biotite-garnet grade) with a regional S1 schistosity subparallel to the original sedimentary structure S0 and a secondary nonpenetrative S2 foliation. New detrital zircon U-Pb geochronology shows that maximum depositional ages for detrital zircons are Devonian, ages of 379 ± 4Ma. These results suggest that the Cerro Negro basement rocks are the youngest basement in the Deseado Massif, overlapping some detrital zircon ages in the eastern Andean Metamorphic Complex in the Andean region. Most of detrital zircons are igneous in origin with a major peak around ~396Ma, probably sourced from the Devonian granitoids of the Río Deseado Complex (El Laurel and Bahía Laura granites) and equivalent northern Patagonia granitoids (e.g. Colan Conhué and Lago Lolog granites). Secondary peaks correspond to Ordovician to Silurian ages, being the Río Deseado Complex and La Modesta Formation (and their igneous contributors) the possible sources of the zircons. The minor oldest peaks yield Cambrian-Neoproterozoic; Mesoproterozoic and Paleoproterozoic-Archean ages, evidencing a common source from the interior of Gondwana. The results provide new insights about the relationships between the pre-Permian metasedimentary rocks of the extra-Andean and Andean region during Mid-Paleozoic ages.


INTRODUCTION
The Patagonia region comprises a vast area South from the Río Colorado river (39°-40°S), extending between the Pacific to the Atlantic coasts (65-72°W) in the southern portion of South America (Ramos, 2008).Patagonian basement outcrops are mainly exposed along the eastern side of the Andes, and can be grouped in two distinct massifs: the Somún Cura to the North and Deseado to the South; separated by Jurassic to Tertiary basins (Fig. 1).Both massifs are characterized by an extensive volcanic/sedimentary cover of Mesozoic and pos-Mesozoic age.Most of the previous studies were performed in the Somún Curá Massif, in northern Patagonia (e.g.Pankhurst et al., 2003;Varela et al., 2005;Pankhurst et al., 2006;Varela et al., 2007) where During the last 20 years many authors argued about the geological origin and tectonic evolution of Patagonia in terms of its autochthous or allochthonous nature with respect to the Gondwana supercontinent (see Pankhurst et al., 2003;Ramos, 2008).The reconstruction of the plate tectonic history of Patagonia during the Paleozoic shows the existence of several episodes of fragmentation and rifting, convergence and accretion, renewed periods of rifting and re-accretion to the Gondwana margin (Ramos, 2008).Furthermore, this author proposes two magmatic arcs for Patagonia during Paleozoic ages: the Western (>401-320Ma) and the northern (310-285Ma).The first obliquely crosses the entire southern Patagonia from North to South through the Deseado Massif, and is represented by the Río Deseado Complex rocks.In the southern Patagonia, at the western side of this arc, several forearc/foreland diachronic basins were developed on continental crust beginning in the East with La Modesta Formation and continuing to the west with the Eastern Andean Metamorphic Complex (EAMC, partly developed over the ocean floor) from Early Paleozoic to Cretaceous times.Such metasediments became part of the accretionary prism of SW Gondwana during Late Paleozoic times (Hervé et al., 2000;Giacosa and Márquez, 2002;Moreira et al., 2005).
This article provides the characterization of a new non-outcropping basement occurrence at the Cerro Negro district, westward of the Paleozoic proposed arc, in the Deseado Massif, and presents a new U-Pb SHRIMP dating of detrital zircons for a previously unknown Devonian basin.The results are compared with other basement units from both, the extra-Andean and the Andean region of southern Patagonia, and providing a geochronological and geotectonic constrain for the evolution of Patagonia during Mid-Paleozoic times.  1 3 4 4 / 1 0 5 . 0 0 0 0 0 2 0 8 2 C .P e r m u y -V i d a l e t a l .
Linkages between the southern Patagonia Pre-Permian basements 139

Extra-Andean region
Basement rocks in the extra-Andean region of the southern Patagonia are exposed in the Deseado Massif geological province (60,000km 2 , Fig. 1).These exposures are small and scattered occupying less than 0.1% of the total area of the Massif (~50km 2 ) mostly covered by extensive Jurassic volcanism of the Chon Aike magmatic province (Pankhurst et al., 2000).In the eastern part of the Deseado Massif, five dispersed occurrences has been gronped in as Río Deseado Complex (Figs. 1; 2A-C).Guido et al. (2004) described the main Río Deseado Complex lithologies distinguishing low to high grade metamorphic rocks (phyllites, quartzites, amphibole and mica schists, amphibolites, marbles, gneisses, and migmatites) which had been intruded by granitic and gabbro-dioritic.Available geochronogical data for the metamorphic rocks of the Río Deseado Complex are scarce.Pankhurst et al. (2003) obtained U-Pb SHRIMP detrital zircon age of ~565Ma from low grade metamorphic rocks from the Dos Hermanos phyllites.These rocks also records Mesoproterozic detrital zircon ages associated with the Gondwana construction though protoliths from older basement rocks possibly correlated with the Malvinas plateau (Ramos, 2008).It has been considered that regional metamorphism occured during the Neoproterozoic-Cambrian periods (540 ± 20Ma amphibole K-Ar ages; Pezzuchi, 1978).
In the western part of the Deseado Massif, La Modesta Formation is known only in restricted basement outcrops in La Modesta-La Josefina area (main locality) and in El Tranquilo-La Bajada area (Fig. 2D, E).Moreira et al. (2005) describes that the low-grade metamorphic rocks at La Modesta-La Josefina area are composed by alternating pelitic and psammitic muscovite-chlorite schists and metaquartzites, with minor calc-silicate rocks, basic metavolcanics and exhalative rocks (graphitic tourmalinites, tourmalinebearing schists and Fe-Mn nodules).The metamorphic grade rang from prehnite-pumpellyite to greenschist facies.These authors also suggest a sedimentation age for La Modesta Formation older than 413 ± 17Ma on the basis of Rb-Sr whole rock study.Recent U-Pb SHRIMP data on detrital zircons yielded 446 ± 6Ma for its maximum age of sedimentation (Moreira et al., 2013), suggesting that the basin closure occurred during Lower Devonian, before the exhumation of the Middle-Devonian granitoids of the Río Deseado Complex.Many of these detrital zircons record Ordovician ages, with a prominent lower Ordovician peak at approximately 473Ma.The oldest analyzed detrital zircons record minor peaks at Neoproterozoic-Cambrian and Meso to Paleoproteroic and Archean ages.
At El Tranquilo-La Bajada area, La Modesta Formation is dominated by metasediments composed of a homogeneous sequence of pelitic and psammitic quartzmuscovite-chlorite schists, with blastesis of biotite and variable amounts of carbonate, feldspar, garnet, epidote and tourmaline (Moreira et al., 2012).These authors indicate that the metamorphic grade corresponds to a biotite-garnet grade within greenschist facies and were interpreted as a metamorphosed pelitic and psamo-pelitic marine sedimentary succession.La Modesta Formation has a regional metamorphic foliation, S 1 , subparallel to the sedimentary stratification S 0 .The schistosity S 1 was deformed by a second deformational event, generating a non penetrative S 2 /L 2 , best evidenced in El Tranquilo-La Bajada outcrops (Moreira et al., 2005(Moreira et al., , 2012)).

Andean region
In the southernmost Chilean and Argentine Andes, the basement of the Cenozoic mountain range is mainly composed of metasedimentary rocks of late Palaeozoic to early Mesozoic age belonging to the EAMC and equivalent units (Fig. 1; Hervé et al., 2000;Giacosa et al., 2012).

Shear zone
Major Road

Creeks and lagoons
Sense of shear sedimentation age recorded for the EAMC, and determined by Hervé et al. (2003), drops by Ordovician age (VS11A: 457Ma).VS11A sample, is located in the easternmost area, and could correspond to La Modesta Formation rocks incorporated in EAMC by tectonic processes (Moreira et al., 2013); it is noted that the oldest detrital zircons yielded a typical Gondawanan signature with a wide range of ages of provenance with prominent peaks in the Cambrian to Neoproterozoic, Mesoproterozoic, and minor Paleoproterozoic and Archean ages.The southern Patagonia metasediments were tectonically included into subduction complexes that were accreted to the margin of Gondwana in late Palaeozoic to late Mesozoic time (~267Ma zircon fission-track ages, Thompson and Hervé, 2002).

ANALYTICAL METHODS
A representative sample of Cerro Negro basement rocks was selected for this study.The U-Pb SHRIMP geochronology on detrital zircons was performed at the Research School of Earth Sciences, Australian National University, Canberra, Australia.The whole-rock (X-ray fluorescence) and trace element (ICP-MS) geochemistry was carried out by the ALS laboratory (www.alsglobal.com).
Zircon grains were separated using standard crushing, washing, heavy liquid (Sp.Gr. 2.96 and 3.3) and paramagnetic procedures.The zircon-rich heavy mineral concentrates were poured onto double-sided tape, mounted in epoxy together with chips of the reference zircons (FC1 Duluth Gabbro), sectioned approximately in half, and polished.The grains were photographed in reflected and transmitted lights, and cathodoluminescence (CL) images were produced in a scanning electron microscope in order to define suitable regions for the analysis.Zircons on the mount were analyzed sequentially and randomly until a total of at least 60-70 grains for the sample was reached.Crystal rims were preferentially analyzed, to date the last growth stage of each zircon.Each analysis consisted of 4 scans through the mass range, with the FC1 reference zircon analyzed for every five unknown zircon analyses; SHRIMP analytical method follows Williams (1998, and references therein).The data have been reduced using the SQUID Excel Macro of Ludwig (2001).The U/Pb ratios have been normalized relative to a value of 0.01859 for the FC1 reference zircon, equivalent to an age of 1099Ma (see Paces and Miller, 1993).Uncertainties given for individual analyses (ratios and ages) are at the one sigma level.For zircons older than 800Ma, 207   Williams (1998).Tera and Wasserburg (1972) concordia plots, probability density plots with stacked histograms and weighted mean 206 Pb/ 238 U age calculations were carried out using ISOPLOT/EX (Ludwig, 2003).

Basement mineralogy and structure
Basement rocks were collected from several drill holes (e.g.VDD-11009, 70º15'W, 46º52'S) below a 50 to 150 meters thick cover of Jurassic volcanic rocks during a mining exploration campaign carried out in the district during 2011.The main rock type is essentially a homogeneous sequence of schists (Fig. 3A).
At hand specimen scale the schists preserve relict sedimentary bedding as primary foliation (S 0 ) formed by the intercalation of siliciclastic packages of different color and grain-size.S 1 is parallel to subparalell respect to S 0 and is characterized by tight harmonic folds of centimeters scale.
Schists are composed of quartz, muscovite and chlorite with minor biotite and garnet and variable amounts of K-feldspar, plagioclase, carbonate, epidote/zoicite, and apatite and zircon as accessory minerals.They include quartz veins concordant to schistosity (Fig. 3B).They also show a porfiroblastic texture defined by euhedral to subhedral garnet, K-feldspar and plagioclase crystals with widespread inclusions (Fig. 3C, D).
Matrix exhibits lepidoblastic to granolepidoblastic texture, with two superposed fabrics: a S 1 penetrative schistosity composed by muscovite + chlorite + quartz alignment with minor biotite and a S 2 axial plane foliation that produces a crenulation cleavage, accompanied by biotite blastesis with some retrograde chlorite at the edges (Fig. 3D).Quartz has undulate extinction and has a polygonal granoblastic texture.Detrital K-feldspar and plagioclase present fragile to fragile-ductile deformation evidences such as intergranular pressure solution and reprecipitation in pressure shadows.
The protolith has been recognized as a pelitic and psamo-pelitic marine sedimentary succession, with a metamorphic grade reaching the greenschist facies (biotitegarnet grade).

Whole rock and trace element composition
One selected sample of the homogeneous sequence was analyzed for whole rock major and trace elements D O I : 1 0 . 1 3 4 4 / 1 0 5 . 0 0 0 0 0 2 0 8

Detrital zircon U-Pb ages
U-Pb ages were determined for a total of 70 zircon spots (Table 1).CL images revealed that most of the analyzed zircon grains have euhedral to subhedral shapes, with sizes ranging from 50 to 150µm (Fig. 5).The most relevant population (about 50%) corresponds to euhedral grains with zoned textures ranging from a well-developed oscillatory zonation to broad and faint growth zones (Fig. 5A-D).Only 30% of the total zircon crystals are broken.Length to width ratios ranges from 0.9 to 3.5 (e.g.Fig. 5A,  B), reflecting mainly different crystallization velocities, whereas changes in temperature and composition of the melt cannot be completely ruled out (Corfu et al., 2003).Some of the grains develop a more complex zonation pattern characterized by a complex oscillatory core truncated at the rim; with an orientation that does not conform with the external shape of the crystal (Fig. 5E).Other grains are made of homogeneous to complex core region overgrown by an oscillatory-zoned rim or a thin homogeneous rim.It is important to note that most of homogeneous broken grains correspond predominantly to Proterozoic ages.
The analytical data was plotted on Tera-Wasserburg diagrams as total ratios, uncorrected for common Pb (Fig. 6A).Most of the data plot close to the concordia curve and  so the areas analyzed are dominated by radiogenic lead.Some of the analyzed grains are variably enriched with common Pb, and so the total ratios plotted on Figure 6A are above the concordia curve.This does not indicate that the areas analyzed are discordant, only that they are enriched with common Pb.In addition, Figure 6B shows the relative probability density plots, with stacked histograms of the detrital zircon radiogenic ages for the Cerro Negro sample.Roser and Korsch, 1988).D1= 1.773TiO 2 + 0.607Al 2 O 3 + 0.76Fe 2 O 3 T -1.5MgO + 0.616CaO + 0.509Na 2 O -1.224K 2 O -9.09; D2 = 0.445TiO 2 + 0.07Al 2 O 3 -0.25Fe 2 O 3 T -1.142MgO + 0.438CaO + 1.475Na 2 O -6.861.B) Th/Sc vs. Zr/Sc discrimination plot after McLennan et al. (1993).C) Trace elements discrimination fields for greywackes from different tectonic environments from Bhatia and Crook (1986).ACM: active continental margin; CIArc: continental island arc, OIArc: oceanic island arc, PM: passive margin.EAMC data from Augustsson and Bahlburg (2008), La Modesta Formation data from Moreira et al. (2013).Note: Upper Continental Crust according to Rudnick and Gao (2003).  1 3 4 4 / 1 0 5 . 0 0 0 0 0 2 0 8 2 Linkages between the southern Patagonia Pre-Permian basements 144 from this calculations due to enhanced radiation damage and subsequent Pb-loss (see Dickinson and Gehrels, 2009).
The U-Pb SHRIMP detrital zircon age for the Cerro Negro basement rocks reveals a maximum depositional age of 379 ± 4Ma (Upper Devonian).This result infers that the Cerro Negro basement rocks represent a younger stratigraphic position than La Modesta Formation (U-Pb SHRIMP 446 ± 6Ma) its main locality (~120km southeast), and overlaps the oldest EAMC ages (~330 and ~385Ma; Figs.1; 7) and equivalent units.
The absence of Devonian ages toward the southeast of Cerro Negro district suggests no zircons supply of La Modesta Formation (Moreira et al., 2013).Nevertheless, it is important to point out that the mentioned ages cannot be completely ruled out for zircons of La Modesta Formation at El Tranquilo-La Bajada area where the lithology and structure agreed with the Cerro Negro features (see Moreira et al., 2005;2012).Hervé et al. (2005) reported late Devonian ages in detrital zircons in metasediments of the Esquel and Cushamen formations, located in southwestern part of the Somún Curá Massif (U-Pb SHRIMP ~335-372Ma, Fig. 7).These authors correlate these units with equivalent units of the EAMC, and propose a common source area and depositional basin, although the former displays a higher metamorphic grade (amphibolite facies).Those units also preserved Devonian to Early Carboniferous detrital zircons of probable early Paleozoic magmatic rocks (Hervé et al., 2005;Ramos, 2008).
So far, the Cerro Negro basement rocks do not record their metamorphism ages.However, the metamorphic event could have occurred before Permian ages, based on the metamorphism ages proposed for EAMC by Thompson and Hervé (2002) and Augustsson et al. (2006).These data are consistent with the Carboniferous to Early Permian metamorphism ages proposed by Hervé et al. (2005) for the equivalent units found in the southwestern Somún Curá Massif.The number (e.g.39.1) is the grain spot number in Table 1.Mesoproterozoic (1153 and 1397Ma), and Paleoproterozoic to Archean ages (1874 and 2543Ma) could be interpreted as inherited igneous zircons coming from the eastern Deseado Massif basement.These oldest zircons were also known in the northern Patagonia and EAMC metasediments, indicating a widespread source area from SW Gondwana to these basins (see González et al., 2002González et al., , 2011;;Pankhurst et al., 2006;Naipauer et al., 2010;Varela et al., 2011;Uriz et al., 2011;Chernicoff et al., 2013;Ramos et al., in press).
Linkages between the southern Patagonia Pre-Permian basements 147 basement rocks indicate that the Cerro Negro paleobasin received higher amounts of igneous recycled materials from nearby sources because of poor sediment recycling processes.

Geotectonic setting
Detrital zircon age distribution patterns offer a powerful tool to determine the tectonic setting of the basin in which they were deposited (Cawood et al., 2012).The overall spread of ages is a function of the nature of the source and the area of the distributive province, with large hinterlands more likely to provide a variety of source ages.
The Cerro Negro basement rocks present an unimodal detrital pattern with a major peak close to the estimated maximum sedimentation age, most likely reflecting a forearc and trench basin characteristics according to the scheme of Cawood et al. (2012).Moreover, these authors proposed a tectonic setting discrimination diagram based on the difference between the measured crystallization age (CA) for a detrital zircon grain and the depositional age (DA) of the succession in which it occurs.The Cerro Negro basement sample shows a detrital zircon pattern falling within a convergent field with a high proportion of detrital zircons (generally greater than 50%) and ages close to the depositional age (Fig. 8).When compared to La Modesta Formation (Moreira et al., 2013) and EAMC (Hervé et al., 2003;Augustsson and Bahlburg, 2008), the latter tends to show also a convergent setting, but with a higher proportion of older ages with respect to sedimentation age (Fig. 8).
This results lead to a forearc or foreland hypothesis for the tectonic setting of the Cerro Negro basement rocks in terms of Patagonia during Devonian times, that later on were accreted to the SW margin of Gondwana.The geochemistry also supports this hypothesis, as most all trace elements suggest a continental island margin in a similar way as La Modesta Formation, whereas the EAMC displays a more complex signature (Fig. 4C).
This idea was already proposed by Moreira et al. (2013) for La Modesta Formation during Ordovician times.Nevertheless, a few thinks should be taken into consideration.The first one is that the arc position during Devonian ages is not well constrained.On this matter, Pankhurst et al. (2003) propose a possible continuity of the Devonian magmatism from northern Patagonia into the Deseado Massif.These authors also point out that the lack of geochemical data for this magmatic suite precludes the assignment of a clear tectonic environment, although some εNdt and Sm/Nd age models suggests subductionrelated arc magmatism.Furthermore, Guido et al. (2005) describe the Bahía Laura Granite as a peraluminous, two-mica monzogranite, interpreted as post-collisional granite.Augustsson and Bahlburg (2008) analyzed the EAMC in terms of provenance, and concluded that the northern EAMC (~48°S) metasediments (with Devonian to Carboniferous depositional ages) were derived from the metasedimentary country rocks of the evolving magmatic arc.
Finally, the available data show that the possible igneous sources of Devonian age for the Cerro Negro basement rocks are located at least 200 to 250 kilometers away from the paleobasin (Fig. 7).For such distances, a major proportion of hinterland zircon sources and rounded and/ or broken crystals should be expected in the Cerro Negro paleobasin.Thus, one possibility is that the Devonian arc was closer to the studied depocenter, now hidden below the widespread Jurassic Chon Aike volcanics defined by Pankhurst et al. (2000).This hypothesis could be supported by basement fragments (schists and granites) of unrevealed age found within Jurassic ignimbrites and lavas located in the central to western Deseado Massif (Echeveste et al., 2001;Páez et al., 2010).
Another hypothesis is that selective pathways deposited major arc-derived zircons into the Cerro Negro depocenter, whereas major reworked cratonic sources fed the northern EAMC and equivalent units in a similar way as described by Korsch et al. (2009) for the New England Orogen, Eastern Australia.Further detrital zircon analysis should be conducted on all basement occurrences within the Deseado Massif in order to clarify this hypothesis.

CONCLUDING REMARKS
Reported low grade metamorphic rocks of Cerro Negro, record U-Pb ages from detrital zircons of Upper Devonian times (379 ± 4Ma, maximum depositional age), being the youngest basement occurrence described in the Deseado Massif and overlapping the Devonian-Carboniferous ages of the EAMC and equivalent units.The detrital zircon age pattern reveals a tectonic setting related with a convergent margin such as a forearc for Devonian times, that is supported by the geochemical signature.
Most of detrital zircons are igneous in origin with a major peak around ~396Ma that could corresponds to Devonian granitoids of the Río Deseado Complex and equivalent northern Patagonia granitoids.Secondary peaks are coincident with La Modesta Formation (and their igneous contributors) being a possible sources for the Cerro Negro basement from Ordovician to Silurian zircons.Minor older peaks yielded Cambrian-Neoproterozoic; Mesoproterozoic and Paleoproterozoic-Archean ages, indicating a SW Gondawana cratonic source such as the Paleozoic basements of Patagonia.
These results confirm the generation of diachronic basins mentioned by Moreira et al. (2005), developed on the continental crust close to the magmatic arc, during the Paleozoic, getting progressively younger toward the west.

FIGURE 1 .
FIGURE 1. A) Location of Patagonia within South America (below right) and the position of the Deseado Massif.B) Simplified geological map of the southern Patagonia region (latitude 46ºS to 51ºS) with the location of the Cerro Negro drill-hole and the other basement outcrops, modified from Moreira et al. (2013).BL: Bahía Laura area, DH: Dos Hermanos area, ESL: El Sacrificio-El Laurel area, ET: El Tranquilo-La Bajada area, LMJ: La Modesta-La Josefina area, MLL: Mina La Leona area, TH: Tres Hermanas area.Fm: Formation.The framed areas are enhanced in Figure 2.

FIGURE 5 .
FIGURE 5. Representative CL images of grains analyzed from Cerro Negro sample showing the location of the SHRIMP analysis and the radiogenic age.A-B) Euhedral zircon with oscillatory zoning internal structure.C) Euhedral zircon with broad zoning internal structure.D) Rounded zircon with faint zoning internal structure.E) Euhedral zircon with complex oscillatory zoning internal structure.F) Rounded zircon with homogeneous core rounded by faint zoning rim.G) Rounded zircon with homogeneous core rounded by homogeneous rim.H) Complex core zircon with rim.Scale bar (in H): 100µm.Note:The number (e.g.39.1) is the grain spot number in Table1.
Linkages between the southern Patagonia Pre-Permian basements 146

FIGURE 7 .
FIGURE 7. Sketch map of the igneous-metamorphic basement from Patagonia adapted from Ramos (2008), with the location of Cerro Negro drill-hole and the possibly related ages proposed in this paper.See discussion in the text.

-Pb zr 446 ± 6 Ma
Pb/ 206 Pb values were used for age calculation.Because of the small variability of 207 Pb/ 206 Pb in the Phanerozoic age range, 206 Pb/ 238 U ages were generally preferred for younger zircons.The 207 Pb/ 206 Pb ages were corrected for common Pb using the measured 204 Pb/ 206 Pb ratio in the normal manner, whereas 206 Pb/ 238 U ages were corrected based on 207 Pb as outlined by

Table 1 .
Williams (1998)burg (1972) results for analized zircons Uncertainties given at the one σ level.2.Error in Temora reference zircon calibration was 0.55% for the analytical session (not included in above errors but required when comparing 206 Pb/ 238 U data from different mounts).3.For areas older than ~800 Ma correction for common Pb made using the measured 204 Pb/ 206 Pb ratio.4.For areas younger than ~800 Ma correction for common Pb made using the measured 238 U/ 206 Pb and 207 Pb/ 206 Pb ratios followingTera and Wasserburg (1972)as outlined inWilliams (1998).

TABLE 1 .
Summary of the U-Pb SHRIMP results for analyzed zircons.oz: oscillatory zoning; fz: faint zoning; bz: broad zoning; coz: complex oscillatory zoning; hcor: homogeneous core with oscillatory rim; ccor: complex core with oscillatory rim; hcr: homogeneous core with rim; ccr: complex core with rim; ocr: oscillatory core with rim; h: homogeneous