Regional and local variations in geochemistry and tectonics along and across Central America

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1 Regional and local variations in geochemistry and tectonics along and across Central America Michael J. Carr, Department of Geological Sciences, Wright Lab Rutgers University, 610 Taylor Rd., Piscataway NJ Abstract Three tectono-magmatic systems create volcanism in Central America; the volcanic front, a weak secondary front and the back-arc. Most magma output occurs at the volcanic front along narrow lines of historically active composite volcanoes in strikeslip/extensional settings. Lavas have strong slab signature, are water-rich and plagioclase and pyroxene phyric. A sporadic secondary front occurs about 30 km behind the main front. Lavas have moderate slab signature and are plagioclase and pyroxene phyric. Most secondary front centers are at least moderately eroded but some Holocene activity occurred in southern Honduras. These two magma systems likely originate from slab flux that lowers the melting point in the mantle wedge. Back-arc volcanism occurs in clusters of cinder cones and small shields in strongly extensional settings, such as the Ipala graben. Lavas have moderate to near zero slab signatures. Typical lavas are nearly aphyric with rare olivine phenocrysts. There has been no historic activity but moderate Holocene activity is certain given the youthful morphology of many southeast Guatemalan and Salvadoran cones and lavas. Decompression melting is likely in these strongly extensional back arc regions. Typical volcanic front lavas are calc-alkaline basalts through rhyolites. The mafic lavas of this group have low HFS elements and are locally called low-ti lavas. In contrast, most back arc lavas and some distinct volcanic front lavas have moderate to no depletion in HFS elements and higher TiO 2. They are locally called high-ti lavas, even though most have only 1.0 to 1.6 wt. % TiO 2. At the Nicaraguan volcanic front most volcanic centers erupt both high-ti and low-ti lavas. In Nicaragua the high-ti lavas are highly depleted in K and LREE, indicating a previous episode of melting. Along the rest of Central America there are occasional mafic, incompatible and HFS element rich lavas mixed in with the normal calc-alkaline suite. In central Costa Rica some of these lavas are alkaline. Wherever high-ti and low-ti lavas coexist, it is impossible to derive one from the other by shallow AFC differentiation. Because of the common presence of two magma series at the same volcanic center and even the same vent, it is clear that composite cones can erupt all nearby magmas. It is possible that some high-ti lavas have migrated from the back-arc to erupt at the front. Mixtures of high-ti and low-ti magmas are common, especially in western Nicaragua and therefore it is difficult to separate them. The best places to find the widest range of magma types are small volcanic centers with multiple distinct vents. Large centers are more extensively mixed, probably because of large, long-lived magma chambers. There is extensive geochemical zoning both along and across the arc. These chemical variations arise from changes in the mantle, changes in the strength of the slab signature and variation the type of slab signature, primarily the extent of the hemipelagic sediment component.

2 The first order mantle zoning is the presence of Galapagos-like isotopic signatures in basalts from central Costa Rica and northern Panama. Recent lavas and Tertiary lavas define a field extending toward HIMU. Northwest of this region, the mantle is more depleted and inferred to be close to EMORB mantle in composition. The second order mantle zoning consists of variations in the MORB-like mantle. Very little is known except that the most depleted region is near Lake Yohoa, Honduras. Several unusual aspects of Cocos Plate stratigraphy make the slab signature complex and separable into components: 1. From Guatemala to northwestern Costa Rica the age and source of the subducted slab are similar, suggesting a near uniform supply to the trench. 2. The sediments consist of a lower carbonate and an upper hemipelagic mud. The Ba/La and U/Th ratios change by only a factor of 2 to 5 through these very different sediments. These ratios are suitable for defining the overall strength of the slab sediment signal. 3. The hemipelagic sediments are rich in carbon and U, creating unusual U/Th ratios greater than 2, rather than about 0.35 for normal rocks; thus the hemipelagic sediments are easily fingerprinted. 10 Be/ 9 Be is concentrated in the uppermost hemipelagic section, thus fingerprinting the top of the subducted sediments. The fingerprint for the carbonates is extremely high Ba/Th. 4. The high Sr/Nd of the carbonate creates a horizontal (constant Nd isotope ratio) mantle-sediment mixing line rather than a mix that extends down the mantle array. Across the arc, the intensity of slab signature, as estimated from Ba/La, U/Th, 87 Sr/ 86 Sr, B/La or 10 Be/ 9 Be, decreases behind the front but not in a consistent manner: Cross-arc transects more than 100 km in length occur across southeast Guatemala and central Honduras, both the result of extensive graben formation, related to the strike-slip Caribbean-North America plate boundary. In southeast Guatemala the intensity of the slab signature abruptly drops to near background level just 10 to 30 km behind the volcanic front. In Honduras, the slab signature decreases with distance across the arc in a more or less progressive manner. Along the volcanic front, the intensity of slab signature varies by at least a factor of 4, but does so in a symmetric pattern centered on western Nicaragua. Ba/La and U/Th, which vary only slightly down the Cocos Plate sediment stratigraphy, most clearly show the regional variation. Both ratios correlate well enough (r>0.80) with 10 Be/ 9 Be to be proxies for it and good indicators of subduction signal. Maximum values occur in western to central Nicaragua, Sharp decreases occur to the west across El Salvador and to the east across eastern Nicaragua. The regional variations in slab signature reflect a combination of changes in slab flux (highest in western Nicaragua) and changes in magma production rate (Nicaraguan volcanoes are smaller, suggesting less dilution of the signal). However, estimates of magma production rates are poor. Along most of the arc there is a local (intravolcano) variation. The different flow fields or magma batches at a single volcanic center differ in major and trace element ratios that emphasize the contrast between the carbonate and hemipelagic sections of the subducted Cocos Plate. The ratios that most clearly separate the two sediment sections

3 are Ba/Th and U/La. Lavas with apparently low hemipelagic content (high Ba/Th and low U/La) also have slightly low K 2 O contents (other major elements are unaffected). At well-sampled volcanoes there are binary mixing arrays in Ba/Th versus U/La. Because the hemipelagic sediments carry the bulk of the incompatible elements, removing some or all of the hemipelagic section can generate these arrays. If the sole cause of this variation is change in source, then the hemipelagic section is sequestered, removed or redistributed on short time and length scales. The volcanic front can be segmented in two ways. The geographic locations of the active volcanoes define eight lineaments that are separated by changes in strike and NE-SW steps of as much as 25 km. Less obvious is segmentation based on the distribution of erupted volcanics. The sizes of volcanic centers are lognormally distributed. Along a segment there is a progression from a large volcanic center to progressively smaller ones. Taking the large centers as keys and defining boundaries at the minima in volume yield seven segments based on volume distribution. These two methods of segmenting the arc do not give the same results but there is considerable overlap. The volcanic segmentation has a tectonic/structural control that is being clarified by geophysical research, especially in Costa Rica. Any geochemical variations that can be linked to a tectonic parameter may allow deeper insight into the subduction process. There are some clear changes in geochemistry between adjacent segments. The geochemical offsets and changes are more consistent with the segment boundaries based on geography than the ones based on the volume distribution, examples are shown in Figures 1 and 2. The characteristics of the slab signal and its variations are listed below for each volcanic segment. From SE to NW: Central Costa Rica: low, steady slab signal, no hemipelagic signal except at Poas. Western Costa Rica: moderate, steady slab signal, hemipelagic present and variable. Eastern Nicaragua: Western Nicaragua: El Salvador: strong gradient in slab signal from low (SE) to high (NW); at NW end is the maximum slab signal in Be isotopes; hemipelagic signal present and not variable. strong, steady slab signal, hemipelagic present and variable; maximun slab signal for most tracers lies between Telica and Momotombo volcanoes in Nicaragua. strong gradient in slab signal from high (SE) to low (NW); hemipelagic signal present and variable. Southeast Guatemala: sharp increase in Ba/La and extensive back arc volcanism; hemipelagic signal present. Central & West Guatemala: Sr and Nd isotope systematics perturbed by Paleozoic crust; hemipelagic signal present and variable.

4 150 Nicaragua 100 Ba/La Costa Rica 50 Guatemala El Salvador Distance Figure 1. Regional zoning of volcanic front lavas in Ba/La. Different symbols are based on the segmentation of the arc defined by volcanic lineaments. Country boundaries coincide with segment boundaries. Nicaragua and Costa Rica both have two long volcanic segments. Three smaller segments in Guatemala are given the same symbol. Note the large step between El Salvador and western Nicaragua. The El Salvador and eastern Nicaragua segments have opposite gradients. The other segments have scatter but no clear gradients.

5 15 10 La/Yb Distance Figure 2. Regional variation in La/Yb. (see Figure 1 for explanation of symbols) The geographic variation in La/Yb is roughly the mirror image of Ba/La. Most points in central Costa Rica (purple crosses) are off-scale with the main cluster reaching 30 and one point at 40. Pb isotope data show central Costa Rica has a distinctly more enriched source, similar to that of the Galapagos hot spot. Along the rest of the arc, there are gradients again across El Salvador (boxes) and eastern Nicaragua (yellow, rounded crosses) but opposite to Ba/La. There is a clear offset between the Guatemala (circles) and El Salvador segments. From western Costa Rica to Guatemala the mantle source appears isotopicallysimilar and the logical explanation for the La/Yb pattern is varying degree of melting. Melting is maximum in Nicaragua, where the Ba/La ratio or slab signal is greatest. Thus, high slab flux leads to high percentage of melting and low La/Yb. Crustal thickness has nearly the same pattern. The exception is eastern Nicaragua, where the crustal thickness is equal to that of western Nicaragua and there is no gradient in crustal thickness.